You've gotten lots of great advice, so I'll just throw in the activities we loved on our trip last year. (2 adults, the kids were 13 and 8 1/2 at the time)
On Kauai - stayed on the South Shore at the Grand Hyatt Kauai and stuck mostly to that part of the island (now we'll just have to go back to see the beautiful north shore

) Kauai was our favorite island (enjoyed Oahu and Maui too and would love to go back, but fell in love with Kauai

)
* Kipu Falls Safari Tour with Outfitters Kauai - kayak, hike, zipline, rope swing, ... great time, great guides
* Waimea Canyon - drove up and did some hiking - amazing views!!!
* Luau at Grand Hyatt - our only luau so can't compare. The food was good and we enjoyed the dancers and fire knife. (we were there during the Koloa Plantation Days festival so I'm not sure whether this was the normal show or not)
* ATV tour with Kauai ATV - loved it! Our guides were fun and the scenery is pretty
* Na Pali Coast sunset cruise with Holo Holo - another great tour! You must see the Na Pali coast one way or another, it is amazing!!
* Spouting Horn
On Maui - stayed at the Westin Resort on Ka'anapali Beach. A good location as a base for what we did
* Horseback riding at Mendes Ranch - they were fun and knowledgable (sp??) and the scenery on that side of the island is really pretty
* drove to summit of Haleakala and hit the lookouts - wow!
* snorkeling tour to Molokini and Turtle Town on the Island Princess - that was our last activity of our 2week trip and it was a great way to wrap it all up
Food (we're not big foodies but here's where we ate - can't complain about anything on any of the three islands we visited)
Kauai - Pizzetta in Old Koloa Town; Tidepools at the Grand Hyatt; Brennekke's at Po'ipu Beach
Maui - Leilani's at Whaler's Village; Hula Grill at Whaler's Village, Giovanni's on Ka'anapali Parkway; Cool Cat Cafe in Lahaina
There's tons of great info on tripadvisor.com
Have fun!!