A must-see on any visit to Hilo is Hawaii Volcanoes National Park. The park includes the Jaggar Observatory, hiking trails, Halemaumau Crater, Bird Park and plenty of activities.
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Be sure to plan a morning trip to admire the rainbows that give Rainbow Falls its name. From Highway 19, go west on Waianuenue Avenue about 1.5 mi/2.5 km and follow the signs.
If you're driving above Hilo on Waianuenue, turn right at the fork in the road and continue to the lookout to see Boiling Pots, a stretch of the Wailuku River where the water eddies and swirls in natural whirlpools. (Resist the temptation to swim in the river, because the strong currents and hidden rocks and ledges make it very dangerous.) About 5 mi/8 km farther, off Saddle Road, are the Kaumana Caves — their lava tubes formed during the 1881 eruption of Mauna Loa. (If you want to explore the tubes, take a flashlight.)