Kids are fun. I never thought that I'd like going to Gymboree, but at this point, spending an hour with my son and watching him have a lot of fun really makes my day. It's become a highlight of my week.
Also, to Daris' point, yeah, a lot of people can do stuff when they have kids. It's easier if you live in a place where you can leave them with someone. However, if you live farther away from family/relatives, it's harder, but yeah, you can still do pretty much whatever you used to do.
I find that the main thing that's different is that we don't go to movies much any more, but even if I didn't have kids, I'd probably still go to less movies because I'm just getting cheaper in my old age, and I refuse to pay $20 bucks to watch a 90 minute Kate Hudson movie my wife picks out that is guaranteed to suck, so I will gladly use the kids as a crutch to get out of doing that.