A Port Isabel day across the bay
South Padre Island and Port Isabel are close neighbors, but crossing the Queen Isabella Memorial Causeway turns this into a real outing. Put the Port Isabel Lighthouse and Port Isabel Historical Museum on the same day so you are not making a second crossing just for another hour of history. Check DriveTexas and bridge traffic before departure rather than trusting an old travel-time estimate.
Morning at the lighthouse
The Texas Historical Commission visitor page describes the Keeper's Cottage Visitor Center and the tower climb. Treat those as two different levels of visit: the winding stair climb has age rules and may close for weather, while the grounds and visitor center can still shape a worthwhile stop. Check the current admission and opening information before you cross.
Museum after lunch, if it is open
The Historical Museum is the afternoon suggestion, not a promise of today's hours. Confirm directly with the museum before building the day around it. If it is unavailable, spend more time around Port Isabel or return to the island; do not replace a closed museum with a rushed causeway crossing. Browse nearby Port Isabel for context and choose lunch separately.
This plan adds the two history stops. It does not include a lighthouse ticket, guided tour, meal or parking reservation.