I have had a huge amount of fun lately going through the J labs. Start at jsoftware.com to download the interpreter, then the labs are all built into the interpreter. You can also run it on iOS. The terse code actually means you can type in real programs even on an iPod keyboard.

When I taught people to play frisbee, I used to throw with my off-hand when teaching, because it made me pay attention to the fundamentals I was trying to teach. Learning J had some of that same feel. I felt like a newbie, but when I saw the kinds of audacious programs you could write, I was hooked. Highly recommended for a bit of fun in your spare time.