Beach, wildlife and a look across the bay
Three days lets you see different sides of the area without choosing a new attraction every hour. Use the first day for the Gulf, the second for north-end wildlife and the third for Port Isabel. The order is a suggestion: move the outdoor day if weather changes, and check opening times before settling the museum or turtle visit.
Day one: Isla Blanca and time to settle in
Isla Blanca Park is the named beach stop in this plan. Allow for unloading, changing and meals instead of treating your time on the sand as the whole outing. If the park isn't convenient for your accommodation, replace it with a city beach access that has the facilities you need.
Day two: the north-end wildlife stops
Pair the Birding Center with Sea Turtle, Inc., with lunch or a rest between them. If two admissions or two walks are more than you want, choose one. The nearby Laguna Madre Nature Trail is another outdoor option, not a way into the paid Birding Center.
Day three: Port Isabel
Go across the causeway for the lighthouse and Historical Museum. Confirm museum hours first. The Texas Historical Commission explains the lighthouse's climbing requirements; looking at the grounds and climbing the tower are not the same visit. Don't assume the tower is an indoor rain alternative.
If this is also your departure day, add the flight or drive first and shorten the Port Isabel visit accordingly. My Trip keeps your outside reservations alongside these ideas. None of these suggested stops books admission, guarantees a wildlife sighting or replaces current weather advice.