If you can only do one day hike, make it this one. Do the 4.5-mile loop counterclockwise through an astounding microcosm of the park that descends through fairy-tale ohia forests to a mile-wide, still-steaming lava lake that was filled relatively recently by a fiery fountain spewing 403 million gallons of lava per second (that's a lot of lava).
Kilauea Iki erupted for five weeks at the end of 1959, alternately filling the crater with several meters of lava that washed against its walls like ocean waves and then drained back into the fissure. The lava fountain that formed the cinder pile above reached 1900ft, the highest ever recorded in Hawaii. This awesome sight suddenly turned terrifying when boulders blocked the passage like your thumb on a garden hose, sending a jet of lava shooting across the crater toward crowds of visitors.
To fully appreciate this hike, first watch the excellent vintage film, Eruption of Kilauea 1959–1960 at Kilauea Visitor Center (or online at www.youtube.com/watch?v=WpQyjVEfx5c), then grab a very informative brochure ($2, or download it from the park's website).
Hit the trail before 8am to beat the crowds. The faint footpath across the crater floor is marked by ahu (stone cairns) to aid navigation. Follow them; the crust can be thin elsewhere.