You can learn all kinds of things online for free--through universities like MIT and others' OpenCourseWare, through non-profits, and through various organizations. And yes, at MIT and Yale and Tufts and other colleges that offer this, you can get the information for the same classes that paying students are taking.
You can try your hand at cooking completely new dishes (you can go here, here, or here to browse recipes). Invite some friends over--and have everyone bring a dish they've never made before. If it's tasty, great! If it came out horrible, you have a funny story to tell. Win/win.
Do you want to learn how to knit, but no one you know is familiar with how to do it? Don't have the money for a class? Try KnittingHelp.com--they have instructional videos, articles, and forums.
Learn another language. From French to German to Japanese to Urdu, the BBC has got it covered. (I really should brush up on my French since I can't do much beyond asking for a beer and where the toilet is after five years of study in junior high and high school.)
Actually, it looks like the BBC has various course offerings, so check it out.
The Khan Academy is a great place to learn (or relearn) math, physics, finance, or history. I'm a math dolt and I should really check this out and (re?)familiarize myself with algebra, trigonometry, and calculus. I can understand the basic concept, but trying to actually do the math makes my head explode.