Visit an Aquarium
If you can't go diving or snorkeling to look for fish, you can do the next best thing--visit the aquarium. One of the best aquariums in the state is Maui Ocean Center. Compared to mainland aquariums, it is relatively small, but there are some excellent exhibits. You can walk through glass tunnels and see numerous fish, including sting rays and tiger sharks, swim past you. Or you can sit inside the peaceful room that houses the jellyfish tank, with its calming music and many glowing jellyfish.
Maui Ocean Center
192 Maalaea Rd
Wailuku, Hawaii 96793
(808) 270-7000
http://www.mauioceancenter.com
Look for Waterfalls
If you happen to be on Oahu on a day when it's raining heavily, consider taking a drive along the H3 highway to Kaneohe. On a normal day, the H3 is a stunning drive, taking you through the lush, green Ko'olau mountain range. However, on a rainy day it is particularly beautiful. Oahu isn't known for having a multitude of waterfalls, but on a rainy day along H3 you'll suddenly see waterfalls cascading down the mountains all along your drive. Also, while on your drive, be sure to look up and notice how the tops of the mountain suddenly disappear in the clouds.
H3 Highway
Oahu, Hawaii
Learn about Hawaiian Culture
If you can't be out on the water surfing or paddling because of the weather, consider visiting a museum to learn about Hawaiian culture. Visitors to Oahu should consider the Bishop Museum in Honolulu, where you can learn how native Hawaiians lived and see old surfboards and examples of early outrigger canoes.
Bishop Museum
1525 Bernice Street
Honolulu, Hawaii 96817
(808) 847-3511
http://www.bishopmuseum.org