Hawaii is home to whales all year long but the famous humpbacks migrate here from December to May and really create a show. They can be seen from shore, or you could take a whale watching cruise. Dolphins are also easily seen from shore, sometimes in large pods of 40 or 50. If you do go out on a boat you may also be lucky enough to have a dolphin or two try and catch a free ride in front of your boat for an up-close viewing. Try the active lava flow on the Big Island, or the Na Pali coast or Waimea canyon on Kauai.
Waterfalls are always a visual treat, along with the many valleys on all islands.
On the Big Island you can actually camp at various places on the active volcano. About three years ago there were two campers who hiked out to an area of the volcano where no lava had flown for many years and pitched their tents near the vent. During the evening an earthquake woke them up and they looked out the tent flap to see the vent, a few hundred feet away, overflowing with red hot lava. At first light, the Hawaii Volcanoes National Park personnel, who had checked the camping permits for that area, flew in with a helicopter to evacuate them. It is about a 38 mile ride, all downhill and can take 4 hours.