On Kauai (haven't been to Maui yet), make sure you go to Waimea Canyon and do a tour of the Na' Pali coast (it is breathtaking). You can only see this coastline by helicopter or boat... we went by boat and also got a great look at a bunch of spinner dolphins and a beautiful sunset.
We went to the town of Hanalei to see the beach there. This is supposedly where the inspiration for "Puff, the Magic Dragon" came from. Regardless, the beach is beautiful and it's a quaint little town.
Poipu Beach -- we stayed at a resort just across the street from this beach. It is supposed to have some of the best snorkeling on the island. I will say that it was great. However, the beach is pretty small, but if you're there for the snorkeling, that's not really an issue.
While you are at Poipu Beach (if you are not staying on this part of the island), drive about 10mins away and see Spouting Horn. It's a pretty cool site. You won't stay too long, but it's worth a few cool pics.
Also, go see Wailua Falls. This is the waterfall that was on the opening credits of "Fantasy Island" (if you are old enough to remember that TV show). Just a bit of pop culture for you. And it is a very easily accessible and beautiful waterfall, too.
Oh, one more note, most everything on Kauai closes down early. We would be heading out for a late dinner (around 8pm) and have a hard time finding somewhere that was not closed or getting ready to close. I guess it really is the "quiet island". Just an FYI. It is, by far, the most beautiful one that we have been to.
All this reminiscing is making me want to go back. Have a great time! Post some pics when you get back.