Learn Python! of all the programming languages, it best balances ease of learning, power and general utility (imho). I started learning it and wrote my first (useful) program the first day and first GUI the second day. It's fairly amazing how quickly you can 'get going' with it. It's available for most if not all platforms and is fairly lightweight compared to most pieces of software these days.
DnD is good fun, you'll need some dice, pencil, paper, eraser and some other similarly inclined people (you generally don't need to buy these). I'd advise pathfinder, because it's awesome and easy to pick up. Most resources are available on-line.
You could learn organic chemistry by getting the good 'ole "Clayden, Greeves, Warren and Wothers" (aka 'the bible') you can get it used for about 40 bucks.
If you're looking for tabletop action, warmachine is fairly cheap at the entry level (about 50 bucks for a "battle box"). Besides the standard pewter, they even have some plastic models to bring down the price.
Or learn sanskrit. Bitches love sanskrit (obscure penny arcade joke).