Hm. As far as tours go, consider a
helicopter tour. They're available on Oahu, Kauai, Maui, and the Big Island. I guess it'll just depend if you fly out of Maui. Each island has a great aerial tour. However, if you're not into the aerial thing, there's always island tours.
Maui has great adventure tours in addition to the helicopter tours.
Oahu offers a cruise option for touring the island. Personally, I'd rather be surrounded by the landscape or as close as possible. However, if you're more into the great views from a distance, this may be the option for you. If not, there is and island tour that drives around the island. Mind you, this will take up a big chunk of your day but it's always nice to see all of the sights.
The Big Island offers a circle island tour that will take you around for the entire day. This would be your chance to see all of the natural wonders of the Big Island.
Here are some links of interests for your tours:
Oahu Tours Kauai Tours Maui Tours Big Island Tours
In regards to the snorkeling, I'm a bit biased but I would say you should try
Oahu snorkeling and you can't do no better than
Hanauma Bay. If you don't have a fear of sharks, then there's always Shark's Cove on the North Shore of Oahu. For this location, it's best to get there early.
Now that doesn't mean that the other islands aren't worthy of snorkeling.
Maui snorkeling is great at Honolu’a Bay from what I hear. It is also a popular surfing area, so if you're out there with the surfers, watch for the boards!
I've listed them as separate events, but they could very well be part of your tours. I have seen tours that offer snorkeling in addition to the normal tour. My preference would've been to do them on separate days but hey you may like to get it all done at once. The choice is yours, I hope you have a great time in Hawai'i!