Just a couple of miles from Hilo, the Wailuku River plunges 80 feet over a wide cave that is the mythological home of Hina, goddess of the moon, and into an enormous pool. When the light is right, especially in the mornings, bright rainbows emerge from the mist over the pool, which led the Hawaiians to name the place Waiānuenue, or "rainbow water." Over time, and especially because of its easy access, Rainbow Falls has become a must-see stop if you are in the Hilo area, though any visitor should be prepared for crowds and the fickle nature of the rainbow.
There is an obvious viewpoint that is visible from the parking area, but if you have more time to explore, it is worth heading up the stairs for views from the top of the falls. The banyon tree forest upstream from the falls is well worth a quick detour; the canopy is so thick that it will feel like dusk in there. Head toward the river to scamper along the bank and enjoy views from the top of the falls. Of course, use extreme caution and good judgment here, as that is the only thing between you and an ugly fall. Farther upstream you may see folks enjoying some of the Wailuku River's swimming holes. Swimming here has obvious risks, not least of which is the waterfall downstream, and there are no lifeguards here. That said, it isn't prohibited, and it is obviously a popular spot to cool off.
For further exploration, be sure to check out nearby Peepee Falls and Boiling Pots.