Southwest of Hilo in the Hawaii Volcanoes National Park on the Big Island, the Kilauea Iki Trail through the Kilauea Iki Crater is probably the most scenic hike if you are only going to do one. The Kilauea Iki Trail is a 4 mile descent through fern forst into a still fuming crater.
It starts at the Kilauea Iki Overlock in lush, ancient rain forest. It drops down and across the relatively new crater floor past active steam vents. Then it heads back into rain forest. It is a 3 mile loop from the Overlook and takes 3-4 hours. The ancient fern and ohia forest is breathtaking.
Many like to do it counterclockwise. If you do, just stay left at every intersection. When crossing the crater, just follow the rock cairns across the floor.
To reach the trailhead from Hilo, head south on SR11. Near milepost 29, keep an eye out for the entrance to the National Park on the left or south side of the road. After entering the park, turn left immediately towards the Lava Tubes. Continue on this road to the first major trailhead turn out next to Kilauea Iki Crater.
Here are some notes on adventuring in Hawaii. First, unlike mainland forests, the tropical island forests can grow very dense and can mask dropoffs. Plus, the trails can be very wet and slick most of the year (December to February is the main rainy season, but expect rain all year round). Stay on the trail to avoid disorientation. Second, speaking of disorientation, be aware that most Hawaiian topographic maps use the Old Hawaiian Datum. When trying to use the latitude/longitude from those maps with a GPS unit, you will think you are 0.4 miles south/southeast of where you really are. Kind of important in a dense jungle. Third, the bacteria Leptospirosis is found in the local waters and mud. It causes flu like symptoms within 2-20 days of contact, and can be fatal if not treated with antibiotics. Fourth, when hiking about, walking sticks can be very helpful for stablity in the slick conditions, for probing mud hole depths and clearing webs of the tiny crab spider.