I’d open my own catering/party planning business. I’d call it A One Night Stand. Advertising would be “Have your next affair with A One Night Stand.” (Shout out to a good friend of mine who came up with that.)
There’s a business like this in Springfield, Missouri called “1984” and it’s vintage pinball and video game machines. You pay $5 admission for unlimited play, except for some of the pinball machines for which they charge 25 cents. They have a snack bar, and their clientele is mostly college students, tween kids, and middle-aged people on a nostalgia trip. 
The music is all from the early to mid 1980s.
The couple who own it are about 50 years old.
I have actually started building a model of my fantasy business. It is way over my head financially so I started looking for partners a couple of years ago.
An R/V, camping and day use theme park that would center around a village of craftsman, artists and science minded individuals who enjoyed teaming up with others just to swee what they could create. Center of the village would include a replica mueseum as well as a restauraunt and theatre, the theatre would be for the express purpose of putting on plays that could be written rehearsed and acted out in one weekend.
The men might be making primitive bows and arrows or other crafts while the women and children and doing their thing. The main attraction would be the customers interacting with one another . The entire program is based on a theory that when we are creative and social and have a group of peers we can share our accomplishments with we become much more motivated and happy.
If I had the money I think it would be a huge help to the genealogical community, at least in one county, to transcribe all the wills and probate records and sell them at a modest price. I’d also like to go through all the newspapers and create a huge database of all the obituaries and death notices. There are 2-3 newspapers in my area from the mid 1800s up through today that are only on microfilm.
After that I’d create a massive family tree database for all the people in the obits, wills, and other historical records. At the minimum I’d have all the vital stats that I could.
Then I’d take massive surveys of all the cemeteries, with photos of the stones.
Not only would all of this information be kicked into the 21st century, but it would offer jobs too. Now to win a couple hundred million.
I used to have this fantasy about being a kind of silent partner theatrical producer. I would have these fabulous dinner parties and introduce up and coming actors, directors and writers to established actors, directors and writers. I’d attend auditions and rehearsals, but make hardly any suggestions about the production. Eventually I’d get a feel for who was talented or creative or at the very least not wastefully eccentric, and then I’d look out for those people. Great shows would come into existence because of me and my money. Talent would be rewarded and theatregoers would see some great stuff. 
I’d like to open an open air wine beer and smoothie bar next to a whitewater slalom course, a place where paddlers could get out and hang out between runs and/or waves. I know of one location (which I doubt will ever become available) which would also be a perfect home base for long distance river expeditions.
The other dream involves creating a travel show. Each episode involves traveling to some place in the world to (1) explore the area’s culture and attractions, (2) paddle on the rivers and/or attend a fest and (3) demonstrate new equipment and/or techniques. I’m not holding my breath waiting for PBS Travel to call back.
My version of this was medieval only, but castle, surrounding village for shopping and such. The castle and villiage would be minimal power and few if any “modern” vehicles. Horse drawn carts would pick up people from the parking area and bring them in.
Castle would be a full blown hotel/event venue for weddings and such but heavily themed to the point that you have to look for the technology. Of course it would also become the premier location for SCA and Ren Faire events. Pennisc would become my bitch.
I would buy a big old farm with tons of land, ideally one butting up against some state or conservation land, cut miles of mountain bike single track (all skill levels), convert the old barn into a B&B, have a huge garden, have chickens, have BBQ & live music on the weekends, and have the best, most fun mtb destination on the east coast.
There are similar type of theaters in the Chicago suburbs. I’ve only been to Hollywood Blvd. but there is a similar one in a nearby suburb. Hollywood Blvd. is done up like Mann’s Chinese Theater in Hollywood with some art deco embellishments. You can see some pics here. They also serve a meal during the movie, which can be somewhat distracting, but fun as well. They also serve alcohol. Interesting place.
Back in the day (the 90’s) my default response to this type of question was a computer shop that built and sold computers, computer accessories, and games and such. It would’ve been called “Ye Olde Computer Shoppee”.
At this point, though, I think I would lean toward opening a SCUBA diving shop on Maui. I’d have all the latest gear and a couple nice state-of-the-art dive boats (No cattle boats). I’d hire a few divemasters with a lot of local experience and do my best to cater the dives to my customers desires and skill level.
I’d also have a tiki bar/restaurant right next door for the divers to share stories when they get back. The dive shop and the restaurant would be right on the waterfront with spectacular views of sandy beaches and palm trees.
I’d spend my days on or under the water and my evenings drinking while listening to outrageous stories and enjoying the breeze off the ocean.
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A two-mile road racing course within an hour of Los Angeles
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A used book store
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A model railroad layout design service
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A feral cat adoption service, including spaying and neutering
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A design and drafting service for residential additions and remodels–hey, wait; I already do this!
Thank you OP for the opportunity!
I would have a laser-tag style set up in an old empty office building with lots of stairs to fly down, authentic looking offices and cubicles, and a basement car park; so that patrons may live out their Die Hard fantasies. There must be a corn field out back for serious hide and seek scenarios.
I envisage most of my millions would be spent on insurance and lawsuits.
With my fantasy profits I would produce a martial arts movie with realistic fighting scenes - no close-ups! My actors would be of the Bruce Lee/Jackie Chan variety.
Yeah
Air-cooled VW and Porsche-only salvage yard. No vehicle too far gone to be bought and parted out.
Bar/bookstore/personal beach resort! Here’s how it would work:
First you need a tropical beach, something off the beaten track enough to be considered somewhat of a ‘backwater’, I’m thinking Central America-ish, so as to be not too far. Half a dozen simple rooms, with showers and bathrooms, over a large bookstore/lending library/bar. Both bar and bookstore open and close upon my whim!
6 rooms means always room for me and my chums, plus room for a few paying guests to help keep costs down. Lots of tropical fruit trees and flowers all around the joint, of course. And lots of hammocks, surf and paddle boards, beach toys, snorkelling equipment, etc. Finally a place to display my awesome seashell collection!
Oh, and lots of watercolour paints and papers, for endless hours of joy!
And no ‘spa like’ crap either, just simple and basic accommodations, right on the beach. The bar/library would be living room and entertaining area all rolled into one!
Don’t worry, you’re all invited to the first full moon party, I promise!
If money was no object I’d probably buy a large chunk of land in the middle of no where, whack a house on it and do bugger all except hunt for meat, plant trees and veges and just maintain things.
For a business that I wasn’t interested in making money out of, I’d open a restaurant in a poor area. Serve basic food that I felt like cooking and get people to pay what they thought the food was worth or do dishes/help out in the back if they were broke.
Industrial design shop. I don’t have all the skills for turning most of the improvements I think of for things into real products. I’d hire the people I need and start making really good, if niche and expensive, products.
I’d do more writing, and with more time to do it would probably be able to start selling.
Open an old people’s home cum animal rescue centre, near a large park. They get to pet and walk (if physically possible) the animals to keep them active, the animals also get a few more years life. I would open it somwhere warm and cheap, probably Greece or Hungary.
Used bookstore/antique shop in a quaint walking town like Red Bank, NJ. I would want a ground floor shop in an old Victorian where I lived over the store. I would have a basement or attached climate controlled garage workshop for my woodworking where I made toys that I would also sell in the shop also. That would be my dream.
Hmm… I like to feed people, so I’ll open a Soup N Sammich Shoppe - no menu just “take it or leave it” - the soup and sammich of the day is at the cook’s discretion. Oh and of course there would have to be fresh homemade bread for the sammiches, and a fresh homemade batch of cookies for each day too. Maybe even a shop that delivers meals to local businesses. I like that idea too - I’ve often wished for lunch delivery at my workplace on days I’ve forgotten to pack lunch. 
I think I’d probably open a microdistillery and specialize in whiskey and aged rum, and not in vodka and gin like most other ones.
The big cost is in the aging- most microdistilleries can’t afford to have a significant amount of production tied up in aging, so they make stuff like vodka, gin and white rum since those require no aging.
I might also get dual-duty equipment and set up as a microbrewery as well, and specialize in REAL Bohemian style pilsners.
Catering Company! I am told I would do well in the catering business.
Things like empanadas ,cakes, fancy sandwiches and a lot of Indian,Mexican,and Korean cuisine.