We took the "strenuous" trail to see Rainbow Falls and Turtleback Falls. It was so worth it! The falls were beautiful. We played in the water near Turtleback Falls but they advise you not to get close to Rainbow Falls as the water is too rough.
You definitely need to be in good shape to visit. My husband and I are in average shape and our calf muscles were killing us a few days after the hike. The kids 5, 6, and 13 did just fine! It is a three mile hike up and down a mountain trail.