The Upper River Trail runs along the banks of the Pedernales River in Reimers Ranch Park. You'll get access to some nice swimming areas along the river as well as great views!
One of the best loop hikes you can do right now (the park is adding new trails) is to head down the grotto/canyon hiking trail to the river, staying to the right when the canyon trail hits a Y fork as it goes downhill. Hike along the river bank along the Upper River Trail, following the "Trail" signs until you reach an intersection for the beach - the river trail then goes to the right, uphill to a dirt parking lot. The ADA trail will provide a nice connector back to the rock climbing parking lot, but until that opens, you can walk back along the dirt park road to the parking lot.
