About 20 minutes outside of Mostar is a small town called Blagaj which you can get to by bus. There's a fortress on the top of the hill there (locally known as Stjepan Grad) with amazing views of the surrounding area. The fortress is overgrown and dilapidated but that somehow makes it more worth seeing. You walk up a road (follow the one and only sign) and then hike up the switchbacks which takes a total of about 45 minutes. The trail is hard to find in the brush but the frustration is worth the view from the top and its free Note: keep an eye out for an arrow made out of rocks. We missed the arrow and spent a long time looking for the trail.
Also, there's a Dervish house at the base of a mountain. There are restaurants in the area because of the picturesque views of the cold, turquoise waters coming out of the cave at the base of the mountain. You can rent canoes there, but the water level seemed way to low for canoeing in September 2013.