I forget what the business was called but we went on an early morning (7am) kayak tour thru the pacific and around the edge of the island. We anchored our boats in a little inlet and we went snorkeling for half an hour. The reefs were incredible. tour operator provided a choice of juice or water before we kayaked back to shore. The pacific is really rough and the 2.5 hours of kayaking wore out everyone. We spent remainder of day at beach/pool.
We also went to the Maui Ocean Center. It's really Bice and has a lot of cool marine life on display. If you have time it's definitely worth it.