I have to agree that Kauai is very peaceful and laid back. It was everything that I pictured Hawaii to be.
When we went a few years ago, we arrived in Oahu in the afternoon, and spent 3 nights there. It is very busy with lots to do and see. Among the activities we did were, surf lessons
http://www.hawaiianfire.com, swimming with sharks near Northshore
http://www.hawaiisharkencounters.com/, and shopping. Due to a screw up with our scheduling of the surf lessons, we missed out on a trip to Pearl Harbor. I will warn you that there is some controversy regarding the shark thing. The state is has laws against chumming the water for sharks, and they are claiming that their boundary reaches 12 miles out because it's between islands, while the operator is claiming it reaches 3 miles out per some other federal laws. The operator takes the tours out 3 miles, which is where the controversy lies.
After our 3rd night there, we took the earliest flight we could get and went to Kauai, and stayed 2 nights there, although because we arrived so early, and the flight back home was at 9pm, we effectively got 4 days on the island.
We stayed at the Princeville resort which was great, but expensive. Like Kyle_98 said, Princeville, and Hanalei are great. I really enjoyed the small towns throughout the island. The northside is much more lush and green, while a greater percentage of the population lives on the south and east side.
The activities we did on Kauai were, Luau, surf lessons in Hanalei, Helicopter tour, lots of driving around checking out the scenery and waterfalls, some shopping.
The things I wish we would have been able to do were, Pearl Harbor, take the boat tour to the Na'Pali Coast, see Waimea Canyon up close (we saw the last 2 from the helicopter at least).
I'd also like to be able to visit Maui and the Big Island as well.
All it takes is money.