This list has some great suggestions. Personally, I suggest games. I’ve spent many an hour staring at the trading screen in Madden 2004 or optimizing my cities in Civ III without once having to look at the Internet. Books are also a good choice, but I usually find I’m not in print-reading mode if I’m noticing the Internet is down.
TV, not so good. Using the TV as a substitute for more interactive activities you’d rather be doing tends to lead to lots of fiddling around with the DVR, which can be irritating to those attempting to watch with you.