1. The Maui Ocean Center is a fantastic attraction to visit, as much of it is indoors and it wont cost you an arm and a leg to visit. Conveniently located in Ma'alea you can easily visit from Kihei / Wailea or from Lahaina to Kapalua.
2. An evening show at Ulalena takes you on a journey of Hawaiian history. Many would agree the show is of similar caliber to cirque du soleil. My bet is you will forget all about the weather outside, be wholeheartedly entertained, and learn a bit of history though this song and dance riddled show.
3. Take a spa day! Maui is home to some of the greatest spas in the world. I worked at the Grand Wailea Resort for 5 years and was lucky enough to visit the spa there as an adult and as a kid. On a sunny day you may feel a bit of guilt being in a spa, but on a rainy day, its absolutely perfect.
4. Drive and keep driving. Visiting the Iao Needle or the road to Hana can be best viewed during a rainy day. No one likes a dry riverbed, but with a little rain, the spectacle is worth the downpour.
5. Just accept it! Rain is coming down (maybe some wind), but its a warm Hawaiian rain. Enjoy a beach walk with no one around. As someone who was born and raised on Maui, I love the beach on a rainy day. A desolate beach on a rainy day is a peaceful place to reflect and rejuvenate if you're willing to take on a little weather. You packed a long sleeve, so go buy a $3 rain poncho at the General Store for you and the rest of the family and head out. If all fails and you get cold, you always have a hot tub, or warm shower to come back to.
Regardless of what you choose, you will have done an activity, and thus, you have concord the rainy day.