I went last September, had a great time. We went to Kaua'i first and then to Hawai'i island...I preferred Kaua'i, it's a lot more lush, and green. Hawai'i is more barren, as it's still volcanically active...that's not to say it's not interesting, just once you've seen a few lava flows, you've seen them all! But it's interesting to see Mauna Kea and Mauna Loa and go into the national park there. I personally think Kaua'i is more beautiful.
Oh and you must go to Roy's restaurant, both Kaua'i and Hawai'i had them, and O'ahu has one as well...the food is amazing! There is some traditional Hawaiian food, but also some non Hawaiian food. The fresh fruit is great, especially pineapple!
As for what's there to do...I'm not sure if you're sporty or not, but surfing's worth a try! The best day for me was on Kaua'i when we took a boat trip round the Napali coast, we saw loads of dolphins, the coast line was amazing, we went snorkelling, it was just a great day. On Hawai'i we got to swim with dolphins at the Dolphinquest centre which was really good as well. As for other things to do, just driving round the islands is interesting enough, there are valleys and canyons galore!! Hope this helped and that you have a good time.