I do a bit of web surfing. I definitely prefer to use a computer when I'm near one. But I'm checking: e-mail, Facebook, Twitter, RSS feeds; every 15 minutes. Viewing occasional videos on YouTube (in high quality mode), and listening to about a half hour of streamed music from Rhapsody every other day while I go out for a walk.
I hardly use WiFi at all, but if you do have a wireless router at home, that'll keep you from using the data side of your mobile service any time you're in range.