I collect fat pencils. My oldest is from 1964. The fanciest is from Russia and has a matryoshka built into the top of it. My favorite is a plain green one that my sister gave me. She wrote “Wisconsin Dells” on it in Sharpie.
I also curl and am learning the guitar. And I have bought an Etch-a-Sketch with plans of mounting two motors on the dials and controlling them from a computer.
I sing barbershop, collect pocket knives and watches, and grow rare plants (Not that kind, folks!)
I restore & operate vintage rail equipment. Not individually, but as part of a non-profit historical group. Been volunteering with this bunch since 1977. A fascinating hobby, if not exactly a “leisure-time activity”. Also, like the OP, I am fascinated by old buildings and industrial archaeology. I’ve spent a great deal of time prowling around old mines, abandoned homesteads, hiking abandoned railroad grades etc. in the western states.
SS
I collect casino chips, which is not all that unusual.
A friend of mine is into rock balancing, which is a tad unusual.
I collect photographs of lost or discarded gloves.
I have a bit of a tendency to use machines for something not quite their intended purpose (I use my sandwich toaster to recycle plastic, for example. I recently had to turn down the offer of a free chainsaw because the temptation is too great to use it for something outside of its normal scope of use - and this would probably be hazardous
I decided to learn how to play guitar in my late 40s. Unfortunately, that was about the time decades of factory work caught up with my hands, especially my thumbs.
I’m a long-time knife collector, nearly all of them folding pocket knives.
I have a small tomato-and-herbs garden. Now that I got my knees replaced, I might expand that.
Hmm…I like to forage for wild herbs and greens, and love reading about them. I’m thinking of expanding that into botanical sketching, Victorian-style. I did art pottery restoration for a time, which was fun in an OCD way. I took up ballroom dancing in my late 20s and learned at least the bronze-level figures in most styles, which led to me teaching some beginner’s classes for a while (I’m dying to get back into this, but moves, life, and health have precluded it). Some dorky stuff, like old emulated computers and games. I like to collect and read old cookbooks (the Kindle has been nice for that), and sometimes make recipes; I’ll be making Florida water soon.
You win. 
Seriously, please keep us updated on your progress! I would love to subscribe to your newsletter. 
I’ve occasionally played the dulcimer. I put it together from a kit about thirty five years ago and still use the goose feather I picked up out of the yard.
I want to furnish the dollhouses! Not with bought stuff, I want to make everything myself.
I’d like to share poems sometime.
Is the Florida water yellow?
I enjoy the challenge my 1930 Kodak Brownie Model F presents, though I haven’t shot much this winter (the second-coldest since 1895 or something). It’ll pick up in the spring when the car tires are round again.
I’m always surprised with some the images it’s capable of producing — but not so surprised at others (bleh). The miniscus lens is uncoated, of course.
It shoots eight negatives on easy-to-obtain 120 film, but the expense of the film and processing, then burning only eight pix (if all are worth it) onto a CD makes me wish someone would invent digital cameras.
A fishing-boat statue, a bridge, a church, ducks in the afternoon.
I collect paraphernalia about total solar eclipses, especially those disposable glasses that people watch the event with.
I also photograph old European windows, especially with shutters or flower boxes or perhaps a cat sitting in the window. Interesting doors too, and sometimes chimneys. I took thousands of these in 25 towns in southern France. I’ve been thinking of creating a book, if I ever have the time.
I like to visit ethnic grocery stores and buy strange exotic vegetables, stuff that has no English printed on it, and/or appears to combine comically mismatched ingredients. Asians have an odd obsession with mixing seafood in to everything.
Mine is scenic benches in and around Victoria, BC.
I like to shoot old-fashioned muskets.
(at targets, don’t worry)
I also occasionally build robots.
Dental flossing?
Spyderco knives.
I used to go in disguise as a hobby. That is, practicing advanced makeup techniques so that people I know wouldn’t recognize me, even up close. I studied things like how to design and create face prosthetics with liquid latex, and how to make and apply facial hair pieces by weaving hairs one by one into fine, nearly invisible netting.
Then I’d go to places where friends were gathered, and test the disguise. I even went to my own high school graduation disguised in the audience, making a point to be in the background of pictures my friends were taking with their families, unbeknownst to them.
Tip: The most important thing about an effective disguise is changing the way you walk.
Lately it seems that I’ve settled into a sessile life of vegetating in front of a computer screen that most often has a SDMB thread on it, in between sessions of sleeping as much as I can. I’ve been actively working on getting into the habit of sleeping as much as possible (but only rarely using sleep-assisting drugs, though). My goal is to sleep about 20 hours a day, leaving 4 wakeful hours a day which I think should be enough to complete the minimum number of chores and errands I need to do. So that’s my hobby, sleeping. I’m raising it to an art form. One of my role models is Wally from the Dilbert strip.
Now this is really THE thing to do!
In my pre-sessile days, I took sailplane lessons (Fremont, CA), 1975-1976, and accumulated 18+ hours dual, and 14 solo flights totaling about 5 hours.(*) And then . . . And then . . .
I got interested in ice skating that did nothing but that for a few years, and never got back into soaring. One of my life’s decisions that I’ve been kicking myself in the ass for ever since.
(*) I know these statistics because I still have my log book. Separately from all that, I also had about 8 hours of single-engine instruction (Cessna 150 and 172) when I was a senior in high school, but got away from it when I went off to college. I’ve been pretty much kicking myself in the ass about never getting back into that either. I guess you’d have to say that I’ve made a hobby of kicking myself in the ass too. My butt hurts!
Then, there’s this guy, Scott Weaver, who spent 35 years building ONE toothpick sculpture with 100,000 toothpicks.
Article with lots of pics; scroll down a ways to see the actual (brief) article.
One man, 100,000 toothpicks, and 35 years: An incredible kinetic sculpture of San Francisco
Another article with lots of pics
Google Rolling Through The Bay for LOTS more. This thing is world-famous. I’ve seen it at county fairs. Can you imagine transporting that thing? No amount of insurance would help if somebody ever, like, dropped it. :eek: :smack: