At the base of the eponymous falls, near the park entrance, is a popular swimming hole (11am to 7pm Friday through Sunday July and August). At other times of the year follow several trails alongside Buttermilk Creek and the gorge it has carved up to Lake Treman.
The park, which is just 2 miles south of downtown Ithaca, has a campground (from $15 per night per tent site) with a few wooden cabins ($70 per night). The walks and swimming hole are quite beautiful, but the park is near a busy road, so expect to hear some traffic noise.