My wife and I will be heading to Maui in early September for a week. We'll be staying on the western part of the isle (near kaanapoli), and planned on doing the Hana drive one day, plus snorkeling at Black rock.

Anyone have any tips or recommendations on things to do there?

(fyi, we've already been to the Big Isle, Kauai and Oahu, so don't necessarily need to whole Luau thing)

My mother and step father went there for their honeymoon.
Best advice I can give you, is not to "OVER PLAN" your stay.
You have seven days, so plan on spending one day on activities, then another day just relaxing, or doing a light activity. Do that throughout the week so you get the maximum enjoyment out of your vacation.
HAVE FUN!

We spent 3 weeks on Maui in your area and discovered some nice places. I can suggest some things to do and then you can perhaps decide about what you like. For less developed beaches, check out the beaches north of Kaanapa: Kapalua is nice and is the next cove over. The next 2 coves north are more primitive. Paia near the airport is a great place to go watch surfers and kite surfers. Upcountry is very interesting if you decide to go up the volcano, (sunrise is beauriful, wear warm clothing, bring a blanket), Haleakela, companies offer group bike rides down from the volcano. There is a fence made from surfboards and windsurfing boards in Haiku. A helicopter ride is really cool, we went with Blue Hawaiian. Hana is a beautiful drive as well. If you don't want to spend a day driving to Hana and back, you can drive around the north side of the island, one way road with cliffs, scary if you are not accustomed to drop-offs, beautiful views, not much for waterfalls tho. From Lahaina you can go out on an America's Cup sailboat, the afternoon sail has more wind. Don't try and find the "Aquarium" to snorkel. The area is now closed off due to overuse, beside you don't want to trespass on Hawaiian cultural grounds. Try surfing! Watch for turtles, lots to see. Snorkel if you are up for that. We rented a car from Maui Cruisers, cool place that only rents older Nissan Sentras, makes you look more local for leaving your car. Don't leave anything important to you in your car. Locals are friendly, food is good, there is a Safeway grocery store in Lahaina. Maui Tacos, good fast food...Aloha.

Thank you all for your ideas. Relaxation is definately key....didnt want to have a full schedule by any means...just maybe a couple of key things to try....and play the rest by ear.

Again, thank you for all the wonderful ideas !