Been to Maui 2-3 times and the big island 2 times with the third time coming up in November (supplier sends us there along with about 20 other couples from Oregon/ Washington). Maui is beautiful and I enjoyed it (been 6 years since last been there and dont remember a whole lot of what we did). The big islands volcano is a neat place to visit. Take flashlights and water bottles so you can see it at night when the lava glows. As was told above, half the island is arid (lots of old lava fields) and the other half is very green. There is a park in the NW? corner of the island that has a nice waterfall and interesting plants/ flowers. Hilo has a farmers market that we chanced upon that had some delicious foods, will definately hit it again. The green side of the island gets huge amounts of rainfall sometimes that can be pretty amazing to be in. We caught the farmers market during one such rainfall and barely made it out of the city before the main highway started to lift up from the runoff and force it to be closed (30" of rain in 24 hours! but the arid side only got 13").
The volcano, while a bit of a drive, is a must see. We are taking our young daughters this trip and our 10 year old is psyched to see it.
If you go, go to the stores (safeway, costco..) in the area and get some food. We got breakfast items and some lunch items for when we didnt want to pay an arm and a leg for meals at the resort.
Snorkling/ scuba diving is great there and the booze/ snorkle cruises are fun.
I'd really like to go back to Maui, but for some reason (golf courses) the owner who is sending us has chosen the big island again (3rd time in a row we will go there). Oh well, a free vacation IS a free vacation!