A number of beautiful waterfalls grace Bali's landscape, but Gitgit is the highest. A day trip through mountains and Lake Tamblingan ending up at Gitgit for a cooling swim is a great way to prepare for the drive back to the south, if that's where you're staying. The walk to the falls isn't too steep, so it's pretty much accessible to everyone. Hawkers on the way down are a bit pushy, but once you're at the falls they leave you alone.
Other waterfalls worth visiting are Tegenungan near Ubud, and Numnum near Sangeh Monkey Forest.