If you're on the NW side, the playground at Parkview North is really nice. Franke Park is also nice.
While the zoo is open it is GREAT! There are several rides, a carosel, lots of places for kids to climb around and wading pools to splash in. Of course, there are also animals to see. :) There is a petting zoo area that my kids really like.
I've met most of the other moms I know at our church, which has tons of kid-oriented programs, too. If you're looking for a church, I'd be happy to give you the info. Several moms from our church also have a play group that I just found out about. I haven't had a chance to get involved with it yet, but I think it meets Thursday mornings.