We saw quite a few humpbacks and two of them swam under and around our boat. It was relatively inexpensive. December to May is the season for whale watches.
Will drop in half.
We did a horse back ride with stops at a couple swimming holes with waterfalls
There is also a zip line park that was pretty fun
The thing to do there, if any.
Great shave ice at Hee Fat general store.
We did the Smith Family Luau and it was great. All you can eat and drink and a show at the end. Nice gardens to meander in before and the offer a shuttle from/to most hotels.
Fern Grotto boat tour
Horseback riding
Visit Waimea Canyon state park
Helicopter tour (we didn’t do one, follow the other poster’s recommendation
Kauai is the oldest of the islands and is very green and lush. Also more low key than say Maui. More time for beach and relaxing.
Two of the highlights of our visit last year were the Jack Harter Helicopter tour-door off!-90 minute tour (linked) and a Na Pali Coast boat tour with Captain Andy's: https://www.napali.com/ We did the sunset sail and even though it was April, we still did see a whale breach at the end of the whale-watching season.
The helo tour followed record-setting rains on the northern part of Kauai: "49.7 inches fell at Waipa Garden in the 24-hour period ending 12:45 p.m. on April 15, breaking the previous U.S. record of 43 inches on July 25-26, 1979, near Alvin, Texas, from Tropical Storm Claudette," so we saw hundreds upon hundreds of waterfalls that wouldn't ordinarily exist. The pilot was great and the view was breathtaking. Well worth the money.
Edit: The Na Pali Coast is a must by air or sea, and the experience is different from each perspective.