Disclaimer - this information is slanted towards South Maui, doesn't include as much about the West Maui resort areas.
An amazingly beautiful (and popular) beach is Big Beach down in Makena. It's fun to watch the waves and people boogie-boarding - or rent your own (there is one you can use under the bed in the ohana). The turn-off to the larger of two parking areas is a little after the turn-off to Makena Landing. At the north end of the beach you can climb over a hill to get to Little Beach; I think it is the only clothing-optional beach on the island, but most people are clothed. Better to go on a weekday - the parking lots are packed on weekends.
I took my first surfing lesson in Kihei. It was fun. The waves are not as big as in other parts of the island, so it is a good place to learn. I took a lesson at a place near Kalama Beach Park, where there is a protected cove with long but gentle waves.
The dive operator we've been out with almost every year: http://www.mikesevernsdiving.com/ When they are booked solid we go out with Ed Robinson.
This is the dive shop that has good rental equipment at decent prices, and a big free brochure describing all the shore dive spots: http://www.mauidiveshop.com/. They have a small shop in a mall on South Kihei Road south of Aka'ula, but go to the main shop which is a mile and a half north on South Kihei Road.
There are a couple of books and maps in the ohana. "Maui Revealed" is really good - especially for things like the Road to Hana, the trip around the Western half of the island, beaches, snorkeling sites, and hikes. Be sure to bring it with you in the car when you're out. The island is so small that you might suddenly find yourself someplace that you've heard of, and then it's fun to look up in the book what you can do when you're there.
A popular trek is the Road to Hana, with lots and lots of stops along the way with waterfalls and sometimes a pool under the waterfall to swim in, surrounded by jungle. It's worth going even if you only make it part of the way. If you decide to do the whole thing, stay a night someplace just before or in Hana - don't try to do the whole trip there and back in one day.
Another popular destination is the Haleakala volcano, especially for viewing the spectacular sunrise. Remember that it is very cold and windy up there so you have to bring a sweater and jacket. Last time we went, we biked down from the top. The company picks you up and drops you off at the top for sunrise, then forms a protective group to bike down. You stop for lunch on the way. Update 2011: I think all bike tours down the volcano have been canceled because there have been accidents pretty much every year.
You can see whales from just about anywhere along the southern or western coasts between November and March. Be sure to have binoculars with you in the car at all times. I also recommend taking a boat outing to see whales up close, with Pacific Whale Foundation in Maalaea.
There is wonderful snorkeling at Fishbowl south of Makena, but it is tricky to get there (walking 40 minutes on sharp, uneven lava, and hard to find the trailhead). See pictures 1871 to 1876. There are instructions on getting to the trailhead in Maui Revealed. Update September 2007: they have blocked off all the parking spots near the trailhead to discourage visiting Fishbowl; probably not worth the extra two-mile hike from the end of the road to the trailhead. Update 2009: The area is closed off entirely for a couple of years for evaluation, repairs, and reinforcement; may reopen late in 2010.
Easier snorkeling at Ulua Beach in Wailea, Makena Landing in Makena, or on the west side of the island, both south and north of Lahaina. Honolua Bay past Kapalua on the West side has great snorkeling and chances to see turtles, but the water is murky after a storm. A little-known beach (Black Sand Beach) with great snorkeling at the south end can be reached through an orange-painted metal gate (drive through) on the right side between the second Makena Road turn-off and the Big Beach turn-off from Makena Alanui Drive. The sand is a volcanic mix of black, red, and white.
I recommend doing the clockwise loop around the western half of the island one day. Start early and take your time. Maui Revealed has suggestions for things to see along the way. One of the neat things is a place towards the end where ocean water fills basins of lava to form a natural spa - the Olivine Pools.
The Iao Valley is a short and easy excursion, very pretty.
Spend a few hours walking along Front Street in Lahaina - boutiques of all kinds, historical sites, cafes and restaurants. During the weekends there is almost always an art and handicrafts bazaar (local artists) under the huge banyan tree. Sometimes there are special festival events there.
In 2009 new trails were made available in the mountains above Kapalua in the north-west. Mostly very easy hiking. There is a free shuttle up from Kapalua and you can do a short or long loop trail and then take the shuttle back, or hike all the way down (six miles).
Lots of our pictures are on the Web. Might give you some ideas of things to do.
Maui Revealed has a page with a huge number of links to commercial activity information.
A terrific site with info about just about all the natural beauty sites in Maui.
There is a very nice beach walk you can do in Wailea just south of Kihei. It's in the books. It starts at Ulua Beach next to the Renaissance Wailea and goes south for maybe 1 1/2 miles.
Upcountry near the volcano you can zip through the jungle with a harness attached to a cable high above the ground. We tried it in late 2007 and loved it.
A central element of our stays in Maui is to walk down to the beach (Kama'ole I) with light folding beach chairs, wine, crackers, and sometimes cheese to watch the sun set. Sometimes it is a spectacular show that lasts for half an hour. There are often side shows as well - a modern ballet dancer practicing his moves, dogs performing miraculous recoveries of things tossed into the waves by their owners, kids building sand castles.
The Sierra Club in Maui offers many hikes, outings, and volunteer opportunities. Other volunteer opportunities can be found at www.hear.org/volunteer/maui. If you're staying in south Maui, consider spending a couple of hours Monday mornings on the beach with Hoaloha 'Aina removing invasive plant species, cleaning up, improving trails, meeting locals who love Maui and work to preserve its beauty and uniqueness.
We always check if there is anything interesting happening at the Maui Arts and Cultural Center in Kahului (north side of the island).
Current water temperature; supposedly there is no difference at depth from near the surface.
