My favorite mai tai, cheap and easy.
On the south side, if you can't see the sun set over the water, go to Spouting Horn at sunset. We saw a green flash from there a couple of weeks ago. If you want an easy hike, try the Maha'ulepu Heritage Trail. It's mostly flat, but it goes along the limestone cliffs overlooking the water. If you want to try snorkeling for the first time, go to Lydgate Park. It's sheltered, so it's safe. If you're headed to the north shore, take the scenic route around Kapaa that passes by Opaeka'a Falls. There's a scenic overlook on each side of the road there. Another thing to do on the north shore is to walk through the lobby of the St. Regis Resort to the restaurant on the other side. It's not open for lunch anymore, but it has a fantastic view of Hanalei Bay. My favorite activity on Kauai is coasting down the Waimea Canyon on a bicycle. Outfitters Kauai will take you to the top of the canyon, their guide will lead the bikes, and the van will follow so the whole lane will be blocked. They pull over occasionally for pictures, and short lectures on the geology, botany, and Hawaiian culture.
Our favorite guide book is
The Ultimate Kauai Guidebook, available in most book stores.