S.,
There are some good ideas here already - I have always wanted to do a party at Libby Lu, but I am waiting until they are a little older for that. It is so cute, though!!!
There is an Emler indoor pool in Arlington, too. It is near Park Row and Bowen. Also, the YMCA on South Cooper has an indoor pool. It is very economical, too, I think. I think you have to be a member of the Y, but I'm not sure. They have party rooms that you use for the party after they swim.
There is also a place called Jumping Party that has a bunch of inflatables and bounce houses inside. The one I have been to is near Park Row and Bowen, but I know there are others in Arlington, too. My 6 year old, 3 year old, and 1 year old all have a great time there.
The Little Gym and Gymboree places have parties, too.
I hope that helps!!
T.