Yeah, depends on what Island.
I've been to I think (as I went as a teen and didn't pay attention to which island back then) all the islands with the exception to the ones reserved for natives.
Maui is very touristy. Should definitely take a catameran (or however you spell it) to Molakini or the Rainbow Reef to experience that, its fantastic.
Rent a jetski, it is NOTHING like using it on a lake.
The Big Island (Hawaii) you'll want to visit the Volcano park. When we went there wasn't any visable lava but island itself is impressive because you'll be driving through rainforest and BAM! nothing but barren fields of volcanic rock.
Seven Sacred Pools with the road to Hana on the Big Island is worth visiting, make sure you take a seasickness pill because the switchback roads are going to make you want to vomit.
Oahu has Honolulu, and Pearl Harbor. It was okay...I'm more interested in ancient history, and I appreciate and respect our history, but we spent waaaaaay to much time at that place and I wanted to hang myself.
Kauai is by far my favorite. Its not too touristy, and its my favorite for beauty. There are waterfalls throughout the island, you can take boat trips around the island that are inaccessable and they'll take you under them. My interest lies with snorkeling so I spend most of my time at Ke'e beach or Anini beach. Ke'e is easy to find, you take the main highway til it ends on the northwestern part of the island.
From THERE...you can take an 11 MILE HIKE through the Na'apali coast which I have yet to do, but want to. You have to get permission to do it as its kinda trecherous, but I've seen pictures and its breathtaking.
Also, if you golf...you can do that on ANY island and have a great experince. I don't golf, but I've raced golfcarts on a few of them and they're very pretty.
Hope that helps.
A friggin' huge war griffin says, "Dude, stop attacking me."
A friggin' huge war griffin asks, "Srsly, wtf?"