Hawaii Volcanoes National Park on the Big Island of Hawaii is famous for the Kilauea volcano, which could be erupting lava when you visit. The lava flows change on a regular basis. Check with the National Park Service (either at the visitor center or on the web) to see if the lava is flowing, and whether it is accessible. The lava flows generally throw off noxious fumes that can make it very hard to breathe, and seeing the lava also usually entails hiking across broken lava to get to a viewing point.
Watching hot lava isn't the only thing to do at Hawaii Volcanoes. The Crater Rim drive is a scenic drive through the caldera of Kilauea that goes through both rain forest and lava desert. Part of the drive is now closed due to volcanic activity - check with the visitor center for more details.
Several day hiking and backpacking trails criss-cross the park. Many of them may have volcanic fume activity that can make hiking uncomfortable or cause breathing difficulty. Check with a ranger to see which hiking trails are open during your visit because the geologic activity in the park is constantly changing.
