I drive through Manvel all the time on my way to San Luis Pass, one of the largest natural passes in the United States. The fishing is excellent and this is the best time of year to be fishing there. Hire yourself a fishing guide and let them take you to the fish and show you how to catch them. PM me and I can recommend a guide for you. This would be my first stop if I were coming here, but I love fishing.
Other than the Air Museum in Galveston and shooting ranges (lots of them, just get a phone book), there are other interesting things to do in the area. One that almost all tourists hit is the Johnson Space Center (NASA) in Clear Lake which is about an hour from where you will be. Most people aren't super impressed with it when they go but it is still cool when you consider the history of the place. If I were you, I would visit. Another big one is the Alamo in San Antonio, also another place that is interesting when you consider the history and fun to say you've seen, but not all that impressive, really. I would go anyway, the trip from Houston to San Antonio is very scenic and will take you about 4 hours.
Those are the two big ones (other than the fishing) but there are all kinds of things to do. Get on the internet and poke around some. Oh, and the food is great. Eat yourself a great big steak from a proper steak house and enjoy some Texas beef. Go to the Strip House in Downtown Houston
http://www.striphouse.com/ you won't be disappointed.