We are staying in South Padre for a week, and I purchased a 7-day pass for the Birding Center for $15. There are numerous boardwalks that take you past several pools. Two of the bird blinds at the end of the boardwalks are out in the Laguna Madre. You have great views of mud flats, pools, marshes, and areas of sand dunes. We saw dozens of species in a two-hour visit during midday -- I'm excited to go back at sunrise.
The Center itself is very peaceful -- There is a gift shop with mostly souvenir-type items, but also several bird guides as well as a South Texas Coastal checkoff list for $1. From the tower you have a nice view of the entire boardwalk area & out into the Laguna Madre.
If you buy the $15 7-day pass, you have access to the boardwalks from sunrise to sunset, even after the Center is closed. The pass is a plastic swipe card that can be used at a turnstile to enter the viewing areas.
Our children, ages 10 & 13 enjoyed looking for alligators, but were also pretty impressed with the Roseate Spoonbills & numerous peeps, ducks, herons & egrets seemingly everywhere you look.
Great place for both serious birders & families who enjoy nature.