South Padre Island is by far the best beach in Texas. I grew up in the Rio Grande Valley and it's a great place. Family oriented, other than March (Spring Break). From San Antonio, you are only about 4 hours away. http://www.spichamber.com/.
Many activities to choose from. Dirty Al's is my favorite seafood restaurant and Palmettos has fantastic Mexican food. It's also nice to enjoy the sunset at any of the restaurants on the bay side. Manuel's for breakfast...monster breakfast tacos. The service in most restaurants down there is usually lacking but I can deal with that. I prefer a condo right on the beach (so we can put the kids to bed and sit out on the balcony). However, beach front is more expensive. Just look around. There are a lot of condos on the beach side that are not beach front (many are new and nicely decorated). There are tons of photos on the web! Just Google South Padre Island photos. It's really nice, especially if you happen to be there when the winds have been calm (the water is very blue and beautiful). However, it is very often breezy down there and it stirs the sand up in the water and dulls the color for sure.
Btw....they are being pounded by Hurricane Alex this morning so not sure what it's going to be like. Probably fine in a few weeks.
Galveston has not been considered a desirable beach until recently. I haven't been in many years but I know someone who's been to this new double high rise condo down there. They said the beach is very nice. Galveston definitely has plenty of activities as well. I still think SPI is your best bet, however. I'd do a little research on the quality of the beaches in Galveston since Hurricane Ike. I had heard there was a lot of debris hidden under the water, but I'm not sure that's a real issue. ??? I can't imagine it would be a problem in the touristy parts of the island. I would think they would have cleaned up the water.
Have a great time! I hope to get down there before school starts too!