Houston has the Johnson Space Center...I thoroughly enjoyed my visit there a few months ago.
I was actually visiting a friend in Austin on that trip, and that's a great town. Lots of outdoors stuff, great bars, etc. Has a lot of the benefits of a college town, but is big enough that it's not solely defined by that.
We took a quick trip down to San Antonio, and I wasn't impressed. Didn't have much time to spend there though...pretty much just the Alamo/Riverwalk area. Much of the Riverwalk is a regular mall with a cesspool of water at ground level. The Alamo was pretty cool, but being surrounded by wax museums and Ripley's Believe-It-Or-Not tourist traps takes a lot of the luster off of it.