Try archery! It’s taking up lots of my time these days, from actually shooting to learning to make flemish twist bowstrings, fletching arrows, and trying my hand at board bow making.
Or learn a musical instrument. Button accordion is making my brain work in ways it was never designed to, so I KNOW it’s good for me.
Lobsang
If you can stand a pint of Castle Ale every now and then to quench the thirst you have acquired running around Snaefell together with a bunch of people behaving silly I most sincerely recommend that you get in touch with the Hash House Harriers. There is a chapter on the Isle of man called the Sodor and Mann HHH. If you do so you can use my good(?) name as a reference.
Genealogy. You can do much of the research online and for free. You will learn about history and geography in the process, and find out things about your family that will amaze you. It will also keep you occupied for most of the remainder of your life.
I’d highly recommend piddling. It’s great because you can piddle anytime or anywhere. Sometimes I piddle in the yard, sometimes in the garage and quite often at work. I can piddle in Rome, Athens or Muleshoe. Tools are allowed but not required. The benefits are that it’s relaxing, sometimes someone actually benefits and you don’t need a uniform. Plus, it’s legal once you turn 38. People really should piddle more often. I think it’s vastly underrated.
Amateur Radio
The instant thrill of contacting someone on the opposite side of the world from you, the endless hours spent chasing the ARRL awards such as Worked All States, Worked All Countries, Worked All Zones, etc.
The endless drain on your checking account as you buy bigger and better radios and antennas to boost your signal to win those awards.
You’ll soon find out why we’re called HAM operators - HAM stands for Had A lot of Money.
Ric
Ham operator for 21 years
[hijack]
DaToad–I have, since I was about ten years old, madly, passionately, wanted to drive a Studebaker. Mostly a '49 Station Wagon, but I could settle for a 1941 Commander 4 Door Sedan ;).
How much do they generally cost? Are they ever sold fully restored? I know absolutely nothing about fixing or restoring cars, so I’d want to buy one ready to put on the street. Is it okay to actually drive a car that old, or are they generally only trotted out for shows? Can they move fast enough to get on a freeway at legal speeds?
[/hijack]
As for hobbys–get a dog? My dad bowhunts, and I tried in vain for all four years of college to get into the PE archery class, but it always had a waiting list of about fifty people. I think it would be tremendously fun.
Martial arts are great too, I took Hapkido classes for a long time, great exercise, made me feel very confident about my ability to defend myself, and very spiritually enriching, as well.
~mixie
Fishing and Homebrewing. They’re symbiotic passtimes.
How come no one’s mentioned jerking off to Internet porno?
Because Lobsang asked for a pastime and not a lifestyle.
Like Archergal said and learn a musical instrument. The very obvious one is guitar. Very portable, easy to learn, impossible to master, lots of fun. Can plink around with it just about anywhere or anytime.
BTW, if you are mechanically inclined I would suggest an old car or motorcycle. What about an old Rover? There are many firms which make pretty decent aftermarket parts and publish detailed how-to guides. Of course, you will inevitably have the “But honey, many people’s hobbies take up the entire garage!” argument, but that is to be expected.
If you’re at all musically inclined may I suggest learning electric bass?
It’s pretty easy to get up and running as far as musical instruments go.
There’s nothing like playing bass to your favorite songs - well at least songs where you’re able to play the bass part.
I burned a CD of maybe a dozen or so of my “favorite songs to play bass to”. I put on the headphones, tune up the bass, spin up the CD, crank everything up, and, well, it can be really somethin’.
Quite a lot of us like to sing in choirs.
http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=219708
Depending where you are, there may be a university or music school based or other community choir that takes all-comers, or even a church choir if you’re that way inclined (I’m not.) This is a more structured thing than learning a musical instrument on your own, since you commit to a rehearsal schedule. As a plus, you also get to meet new people.
My other pastimes are - cooking, motorcycling, gardening (new, i’ve never had a garden of my own before) and cycling. And annoying the cats.
It’s my favorite. If you ever run out of research, let me know. I’m sure I could find some for you. 
Chess is a fine pasttime, it is intellectually stimulating and fun. You can playit online or probably find some local coffee shop or club tat meets. You can meet others likeyou who will have a defnite opinion on e4 versus d4.
That is an excellent suggestion and I wish I’d thought of it. I started hashing at age 12 with my dad* and it’s an excellent way to get out, meet interesting and fun people, get a bit of exercise, see places you wouldn’t see normally (even in your hometown you’ll be surprised what you’ve been missing) and oh yes, drink a few tasty beers. Definitely worth looking into.
*I didn’t drink beer at age 12, though; my father, iron-fisted autocrat that he is, wouldn’t let me drink until I was 15.
Magic: The Gathering
You did specify a pastime which will expose you to more sweaty young men with no social skills than you previously knew existed, and yet is still strangely compelling, didn’t you?
Nice, but this makes me feel extra warm and fuzzy inside 
Why not combine your wanting to hit targets and lieu’s piddling idea. Sounds like the perfect hobby.