Maui was great.
Yes, deffinitely rent a car. Rent a convertable. The highways that wind up through the hills (particularly along the coast) are beautiful, and your passengers will want to be able to take in as much of the scenery as possible. Along the highways that run along the coast, there are spots where you can park, and hike up to water holes with beautiful water falls. This is something I deffinitely recommend.
As far as snorkeling is concerned, the greatest is to be found on the big island of Hawaii at Kailua Kona. I don't know if you are going to the big island, but the spot at Kailua Kona is considered to be one of the best snorkeling spots in the world. After visiting there - I would agree. It was amazing. This is a spot that is filled with literally dozens of species of tropical fish. You can also get very close to the sea turtles who come to this particular area of shore to feed.
Here is something very important about snorkeling anywhere in Hawaii:
The lava rock and coral in Hawaii can be extremely sharp in most areas. Some of them are razor sharp. Several of us cut our hands and feet when snorkeling and didn't even know it for quite some time. Don't try to snorkel without fins or water socks. Trust me, you will want your feet to be protected from getting cut on the rocks. I went out without my feet being protected and I sliced one of my toes open. When snorkeling, you will also constantly find yourself grabbing onto the underwater rock and coral. This is where you will easily cut your fingers if you're not careful. I would recommend wearing surfing gloves. If not, when grappling onto underwater rocks and coral, grab them with your fingernails, as if you were clawing them. This will save yourself many bandaids!
Also, watch out for the black sea urchins. They look just like little black porcupines. Be careful not to step on them or brush up against them. The spines break off and if they get stuck in your skin, it's exremely difficult to get out; often requiring a doctor to do it.
Make sure your camera has plently of memory storage. You will certainly wind up wanting to take more pictures than you had anticipated.
Enjoy your trip! Post your pictures when you come back!