The laid back pace and the sheer lush of Kauai is what sets it apart. People on other islands go there to vacation themselves. The main highways doesn't even circumnavigate the island completely, it's more of a backwards C. You'll ask "WTF is with all these chickens" when you get there.
Honestly, it's not really the kind of place you should plan to have massive consumption of touristy based outings. The pace is MUCH slower than the other islands. If you make it the north shore you'll wonder if you've landed in some kind of tropical Tofino. Let the ebb and flow of the place guide you and you'll have adventures enough. Your list sounds pretty good to me, the only thing I would add for places to see are the canyon on the south side (pretty spectacular -- you'll look down at it and wonder what the teeny tiny little specks are and then you'll realize they're helicopters -- you're that much higher than they are
), the north shore has that beach near Princeville which is the inspiration for just about any painting you'll ever see of Hawaii and even possibly the inspiration for the fictional Bali Hai -- it's just a pretty place overall.
Snorkeling or diving is a solid "not bad" - beats the heck out of anything in the prairies
.. I can't remember about April specifically but the north shore and south shore alternate seasons for the better snorkeling - the waves get too big otherwise.
The coffee plantation is also pretty cool.