Blanco State Park is a quick trip out of San Antonio, just an hour north of the city. Fishing and swimming are permitted but there are daily entrance fees for adults. You can rent tubes or bring them with you.
Activities include camping, swimming, picnicking, hiking, nature study, boating (electric motors only) and fishing. Tube and canoe rentals are available at the park. The Blanco Classic Car Show is the third Saturday in May.