Kauai is great. Way more laid back than the other islands and considerably less developed (there's only one road outside of the towns and it doesn't even go all the way around the island.
). Where are you staying? The north shore is better in the summer and the south shore better in winter months.
Waimea canyon is worth seeing, they call it the Grand Canyon of the Pacific.
My wife and her sister did a float down these >100 year old irrigation canals that go through tunnels. They take you up the mountains, put you on these inner tube like things and you float through the rainforest and stuff. They had an absolute blast, I can ask her for the info if you like, the name escapes me (I didn't get to go, SOMEONE had to watch the kids. I'm such a hero.
)
Got small kids? There is the coolest play park in this state park just north of the airport. Two actually, one is a bridge that was designed by kids. It's like a 3d maze you can easily spend hours "crossing the bridge." My kids (2 and 4 at the time) just loved it there.
It's wet, it's one of the heaviest rainfall areas anywhere, this means two things: green and lush like you can't imagine, and not as hot as you might come to expect from the tropics. For us when it would rain it would be a torrential downpour lasting maybe 10-15 minutes and then it'd get sunny again. Think the hottest it ever got was around 24 or 25.
The coffee plantation is good if you like coffee
