If you want to swim in a waterfall pool, there are very few places to do that with easy access, but this is it. You can also cliff jump here, if you have the courage. Easy hike, which this video details. Beautiful place. Jump at your own risk.
To get there, you can put Wai'ale Falls in your mapping app, or here are directions. Take Waianuenue Ave, which is the road you take to Rainbow Falls, keep going until it curves right and you cross a small bridge (across Wailuku River). Park at the bridge. On the west side of the road, on the north side of the bridge, enter the trail. Follow the trail west.