Pack shorts and T-shirts and comfortable walking shoes. Sandals are OK as a second pair of shoes but expose your feet to the strong sun. A pair of long pants and lightweight long sleeve shirt will be useful in the field or on cool nights. Although rain is scarce in the summer, perhaps it is still wise to bring along a lightweight rain jacket for that rare thunderstorm. And do remember to bring a beach towel.
Hats are also useful as are sunglasses.
Do try to keep your luggage at a minimum. Rental cars and public transit have limited space. It should be noted that many of Portugal's historic castles and fortresses can be dangerous to those who do not exhibit prudent caution. Unlike many American attractions there are few if any barriers restricting people from getting too close to the edge of sea side cliffs or stepping out onto crumbling castle walls. This can be especially dangerous to kids who might enjoy running along the battlements and ramparts.