I find that if I'm going to the gym to work out, I don't much talk to people - when I'm lifting or on the treadmill or whatever, I'm focused on what I'm doing, and everybody else seems to be in the same boat. However, if it's something like a pickup basketball or soccer game, there's always time when you are waiting around between games that there is time for people to interact and chit-chat. Even more so with something like softball, where half the game you have down-time sitting in the dugout waiting to get up to bat.
Pickup Ultimate Frisbee is one of my suggestions, as you're right on about this. There is pickup Ultimate everywhere and people are mellow and friendly. You can use pickupultimate.com to look up local games (it's not complete, but a good start). People will happily teach you how to play -- just stand on the sideline & ask questions. Bring a white shirt and a dark shirt and you're ready to go.
