What to do on a "rainy":http://www.hawaiiweathertoday.com/?page_id=20 day in Maui, well thank goodness for the Internet:
"Hawaii Show Time":http://www.hawaiishowtime.com/Movies/index.asp?Isle=2. 10,000 BC looks good as does Nim's Island. 1:30 and 1:45 shows or Horton hears a Who at 3:45, but the wind is picking up and "Beaufort scale":http://en.wikipedia.org/wiki/Beaufort_scale#The_modern_scale is 4-5.
Snorkeling is lousy with the rain.