If you could play a harmless joke on someone (individual or even people in general) with no limit on cost what would it be?
This is one I did and it was free. I found an old remote in the sofa so as Mrs Cad and I were watching TV I would surreptitiously his pause at random. Or rewind. Or play when fast forwarding through commercials. The trick is to do it once or twice a night and skip some nights.
Is gaslighting harmless?
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Get all of their friends, family and coworkers in on it and treat your spouse to a power weekend somewhere awesome. Has to be domestic because of passport stamps and prep by taking a couple pictures of the cat being cute and maybe an interesting flower.
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Now when you get back, save the pictures of the vacation they took on their phone on your computer and erase them off their phone replacing them with the innocuous pictures time/date stamp changed to the weekend, as if you spent the weekend at home. Act as if you never went on vacation. They call a friend, “You weren’t in New York last weekend. We chatted on the phone while you were doing laundry.”
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Laugh to yourself until they find out. Once they do, hire a good divorce attorney.
That to me doesn’t sound harmless. A tiny gaslighting (to conceal a future gift or surprise) might be all right, but not to the scale you’re mentioning.
My current favorite are what is called the invisible danger prank.(https://www.youtube.com/results?search_query=invisible+danger+prank) Watching the marks suddenly being scared of nothing in particular can be quite hilarious.
I’m going to disagree on the harmlessness of the invisible danger stuff, only because the victim almost never finds it amusing.
The only worthwhile pranks, to me, are the ones where the prankee shares in the funny when it’s all over.
mmm
As a victim of a harmless prank at work a number of years ago and ended up in the emergency room there is no such thing as a “harmless” prank.
Somebody is going to get hurt, either emotionally or physically.
It’s only funny until someone gets hurt.
Then it’s hilarious! 
Seriously not going to share your story?!
Many moons ago, I had a temp job processing Medicare and Medicaid claims. It amounted to me sitting in a big room with a bunch of ladies opening envelopes and putting the forms into separate stacks.
Most of the ladies had headphones and Walkmans and would listen to The Young and the Restless while working. They were all taping it on their VCRs as well and would watch them when they went home. Sometimes one of them would pipe “Did you hear that?” whenever one of the soap villains would say something objectionable.
One day Belinda, who shared my table, started crying. I asked her what was wrong. She said “It’s Lisa (or whatever the character’s name was). Her kidneys are failing and she’s gonna die!”
So that evening, I looked up the actress and downloaded her picture. I made a fake letter from CBS on my computer, and pasted the picture in it. Below it, I typed “Dearest Belinda, thanks for being my biggest fan. I assure you I’m doing fine. The doctors replaced my kidneys with pig kidneys. Other than an occasional desire to eat slop, I’m doing fine.” I put the letter in Belinda’s drawer when she was away.
For a week or so, she didn’t say anything about it. Then one day while listening to the show, she started sniffling again. I said “Is this about Lisa’s kidneys?” She suddenly slapped the table, pointed at me, and yelled “DID YOU LEAVE THAT NOTE?”
She pulled it out and read it to me. I feigned ignorance.
There were no emergency room visits.
Money is no object?
Me: Hey, I bought you a private island!
Them: Wow, really?
Me: Haha, no, it’s mine.
Them: Oh. 
Me: Your private island is next door! Joke’s on you sucker, it’s only 80% as large!
Us: Hahahaha (champagne glass clinking noises)
I saw a video of a prank in Australia in which friends of the vacationing mark re-plumbed his house so that beer came out of every spigot. Sweet.
Is this a harmless prank (asking opinions)? Mutant Giant Spider Dog, by Sylwester Wardęga (2014) - YouTube
We used to do this in school. In the late 80s the school system switched over from mainly films to TVs and VCRs for in-class viewing. I took an extra remote from home that was the same brand as the school’s equipment.
Teacher puts in a tape and hits play, I switch off the TV. They turn it back on, I change inputs. They start the tape again, I fast forward. And so on. The trick was timing it just as they hit a button so it seemed like the equipment was malfunctioning somehow.
My safety glasses/goggles were greased with machine grease.
My reaction was supposed to be that I take them off now looking somewhat like a raccoon. Ha, ha, ha, funny, funny. Instead I took them off as I felt something near my eye and took my arm/shirtsleeve and wiped across my eyes getting grease in one eye. Luckily it was clean grease as dirty grease is blacker and the results are supposedly funnier which they normally used. Looking back I easily could be blind in one eye due to this prank.
MANY years ago I recall reading of one prank where some cartoonist won a “prestigious award” and his compatriots decided to send a series of telegrams. The first was to read “What does name do when he wins prestigious award”. Then every day another telegram would be sent along the lines of “He gets the flamingo on his lawn bronzed.”.
The real payoff was that the first person forgot to send the kickoff telegram. So then every day afterwards the person was getting telegrams that made no sense whatsoever.
Funny then, creepy today.
Film an alternate ending to Big.
A couple of decades ago, a truly harmless prank was to send a flamingogram. The recipient could wake up one morning to a few dozen fake flamingos planted in his lawn. The craze for doing this lasted less than a year.
I say we bring it back!
I loved the flamingo prank.
When my brother and partner bought a new home in what we called “a Martha Stewart neighborhood,” I bought a whole bunch of tacky yard art such as plastic geese, popsicle style candles, plastic flowers, and yes, flamingos and spread the over the yard early on the day they moved in. Life is good.
Another good prank was when a coworker had been out for a month due to surgery, we did up her work cube to make it look like it was filled with shredded paper. It wasn’t, of course; we weren’t mean. But we glued shreds to the edges of paper and arranged them around the top edges, doorway, and over the window in her cube so it looked like the cube was packed. It was an impressive job, all done on our own time. Taking it down took less than 5 minutes.