So, it's raining. Here are a few things you might consider:
1. Visit the Whaling Museum at Whaler's Village. Admission is free and it's quite interesting.
2. If it's not raining too hard, tour the historic buildings in Lahaina. The guides are quite knowledgeable. It's often dry in town while it's raining quite heavily elsewhere (especially to the north).
3. Take in the art exhibit in the old prison by the Banyard tree.
4. Drive toward Hana. The waterfalls along the way can be spectacular during the rain.
5. Drive to Wailea. It's the dry part of the island and is often only cloudy while it's raining elsewhere. Visit the shops at Wailea and tour the Four Seasons and Grand Wailea hotels. Use valet parking. It's free at the Four Seasons and you can ask a bartender to stamp your parking ticket for free parking at the Grand Wailea.
6. If you like bargains, try Savers in Kahului. You can rummage around and get some really nice clothes at great prices.