Inflatable Wonderland in Rolling Oaks Mall is similar to Pump it Up but it has open play so you can go anytime. On Tuesdays and Thursdays it is half price all day. They also have a toddler area that is good for little ones. Kidzville on San Pedro is a huge indoor playground. They have all the playground equipment indoors along with some bounce houses and slides. I think they do mostly parties now so you may want to call about open play. There is also a new place opening up on Stone Oak Parkway called Big Top Kidz. I believe it will open on July 30th. I don't know much about it other than it is geared towards kids 6 and under.
The Children's Museum downtown is great. Lots of stuff for kids to do. Also the Witte Museum has a very large educational children's area and it is free on Tuesdays after 5. They have a cool dinosaur exhibit through September.
Hope this helps.