There are a ton of good activities and tours on the Hawaiian Islands. I've been on some, but not a whole lot. I try and go to Hawaii once every 2 years. I used to live there.
Here are a couple good resources to help you research. I can't tell you how important it is to do your research before hand! You don't want to get sucked into what ever is around and not have a good experience, especially considering that you're going on all the islands. (i'm going in November!!)
Hawaii Discount Activities
is a good one. They have descriptions and a really big selection.
Maui Vacation Network
has activities as well as info on rentals, cars, restaurants, etc...
is also a good one. This has mostly activities for Maui, but covers other islands too.
I personally would do a boat trip to snorkel. See if you can do a night dive, or snorkel on the big island with Manta Rays. I did this and won't forget it!
Still need info on Luaus.
If you're adventurous, do a doors-off helicopter ride, though the kids might freak out. I would take them to the best beaches
and save some money over doing the heli. Heli tours are better for couples and older kids.
Hope this helps. Have fun!