Haleakala.

Drive to Hana.

Golf at Kapalua Bay.

The most kid-friendly beaches are right there at Ka'anapali.

Snorkeling at Molokini.

Bubba Gump's in Lahaina ain't the greatest eats... but the experience is fun... especiallty for kids.

I don't know if it's still there... but the weekend seafood buffet at the Kapalua Bay Hotel was pretty damned good! Pricey, but good.

I think your idea of just relaxing and letting the activities come to you is probably the best plan. Have fun and enjoy!