I would think that a 3 year old would be very bored on the long drive to Hana and back unless she does really well with car trips that last for hours. Check to be sure that the place where you will be staying has a pool for toddlers. Some hotels do and others don't. Also, there is a beach called Baby Beach on the North end of Lahaina that is very good for toddlers. Ask the staff at your hotel how to get there.
She might like to ride the Sugar Cane Train that goes from Kaanapali to Lahaina.
I think 3 years is too young for snorkeling. But she might enjoy the floatation toys that have a plastic window in them to peer into the water for fish. Of course, plenty of beach toys for sand play is always the biggest hit for kids her age. She probably would enjoy the watching trip. PWF gets good reviews.
Restaurants we love are: Dukes, Hula Grill, Barefoot Bar, Sansei, Mama's Fish House, Star Noodle, Leilani's, Kimos, Pacific 'O.

Bring a small sun tent for the baby. The sun is intense and there is often no shade. We got a small, easy to set up one at Walmart that folded up very small. These are similar:
http://www.tinkertots.com/uvshpr.html?gclid=CKLWrpSojqcCFQN7gwodpBdNeQ

We brought our daughter to Maui when she was four years old. We also stayed in the Kaanapali area. She spent the entire week digging sand and hauling buckets of water to and from the ocean to her hole in the sand. She was quite impressed with the "Big River" as she called the ocean.

We usually set up beach chairs near Black Rock so my husband and I could take turns snorkeling while she played. It was a fun vacation...definitely slower than what we were used to, but still very fun!

I topped my old trip report for you. It's three years old, so some of the information is probably out of date. Re-reading it, I see we spent most of our time at the beach because that is all our four year old wanted to do.