Surfing

There are a brazillian websites devoted to the best beaches in Hawaii. I'm just starting to learn to boogie board and I've found two beaches that have the waves for it near Hilo. First is Richardson's Beach and the second is Honoli'i. Richardson's is the most well rounded of any of the beaches I've been to. It has snorkeling, wading, sun bathing, a grill, showers, and waves for body boarders. Even has a small beach for kids to play in. If you're in a group with mixed interests regarding a beach, Richardson's is the place to go.

The only down side to body boarding at Richardson's is that in the middle of the wave break there are a few big rocks. During high tide you can only see them as a wave goes past. There's intimidating and you don't want to get caught on a wave headed towards them. It happened to J and I and it was pretty scary stuff. We made it with only a few scrapes.

Honoli'i is a famous beach for surfing. It's about the only thing you can do there, the beach is mall and full of rocks. The surf is outstanding. It's perhaps not the best place to learn to surf as there is always a crowd and the waves can get pretty big if you don't know what you're doing (like me).
Both beaches are within biking distance of Hilo.