I was in Galveston in May 2010, it was kind of neat to check out for a day, but I don't think you'd need any more time than that. While there, we stumbled into a BBQ competition that was well worth seeing. Looks like it's on May 9-10 this year:
http://www.ycfund.org/. If you can make it for that, it's definitely worth checking out, even just to see the rigs they have set up and the smell delicious aromas in the air.
NASA is pretty cool, but a lot of it is geared towards kids. The best part for me was the gallery area where they have all the different kinds of spacecraft they've used.