Be sure to check out these fun activities!!! (Be aware that my spelling of these names is very poor. Pick up the "Maui Revealed" guidebook for great fun.
No need for a 4WD unless you are planning some serious off-roading. You can go all the way around the island (all round Hana and west Maui) with no problem. Even the one lane roads and gravel are fine.
Petroglyphs just south of Lainha in Olwalu. These are a short walk from a parking area with native (and not so native) vegetation. The petroglyphs have seen better days, so be careful, there are lots of big rocks hiding in the grasses.
Aloha Mixed Plate in Lahina. Go to this place. It's a great outdoor eatery. Not that expensive and good traditional mixed Hawaiian food.
Stone Mushroom and the Olivine Pools. Be sure to drive around west Maui. You'll find these beautiful pools right where the pavement changes and the road narrows. There are a couple pools just west of the "official" olivine pools that were really empty. They get a bit more ocean movement - so be careful when the ocean is rough.
Last year when I visited Maui I got a fabulous scrape at the Olivine Pools. This year was no different! I was really tempting fate when I was splashing about in this pool and a 15 foot wave crashed in on these lava formations. I was attempting to hang onto the lava when one smashed into me and took me for a rock surf. Luckily I only got a little scrape on my hand, arm and butt. At one point in the surffest I looked down and saw only foam. I thought, hmmm, this is delightful. I popped out of the foamy surf laughing. I am no Birth of Venus - cool and reserved.
The Iao Needle. This was ok. There were kids playing in the streams here which was cool and it was nice to see a canyon, but all in all I was not blown away. I may have been a bit tired.
Pa'ia. I love this town. It's billed as a "hippy" town on Maui and I guess it is. It reminds me somewhat of Haight Street (SF) and Telegraph Ave (Berkeley). First thing I do when I get to Maui is go to Paia. It's only about 10 minutes from the airport. I buy my straw cowgirl hats there (there's a hat place on the right just as you come into town). I got my bikini at Maui Girl on the main drag (turn right at the light and park). And I fueled up as we came back from Hana at Charley's. The burgers and beer were just what we needed after a 11 hour day of driving and fun. Stop here and check out the stores. Support local color.
The Road to Hana. There are a ton of guides for this. Do it backwards. Start out by skirting the volcano (don't go to south Maui) and then drive on the oh so scary roads on the southeast. There are stretches of gravel and badly paved one and one and a half lane roads. These are great roads and if you go early you'll meet only the locals. The upcountry is beautiful with highland meadows with cows grazing and pheasant. We even met a tractor on the road.
The seven sacred pools. It's a national park so you can use your national park pass. Try to get here early because this is a well known place. Maui revealed has a great description of this place and the Pipiwi trail, so use that. Let's just say that you'll not only see amazing waterfalls, guava trees, huge banyan trees and a bamboo forest! It's a great and fairly easy 4 mile roundtrip hike.
My friends and I stopped off under the huge banyan tree to do some tree climbing - it was a huge tree and looked so fabulous for climbing. We noticed some flip flops at the bottom of the tree but we didn't see anyone up in it, so we started climbing it, only to find a couple people having an intimate moment. There's plenty of room for some action up there - but I'm not so sure about the comfort level.
I stopped by the Venus Pool and that was beautiful. And I heard great things about the blue pool. Be sure to take lunch with you on the Hana trip because Hana has not many food options... There are lots of places looking to get exchange your money for a hana packed lunch. ;)
I've still not made it to South Maui or the crater. I've heard amazing things about La Perouse Bay (especially snorkeling at Fishbowl) and the Sliding Sands trail in the crater.... I can only imagine that it must be somewhat like the Mojave, yet completely different.
So, go have fun now!!!