My wife and I have been going to Kauai just about every year since our first trip in 2016 for our tenth anniversary.
Just a frustrating FYI - Kauai county as of last year or so started charging for various park access. You can typically pay by credit card at the kiosk at each location.
I would recommend getting a 4 wheel drive pick up or jeep. You can go a few more places. March my find it cool up in the Waimea Canyon area, be prepared for that.
While in Poipu, make sure you visit Kauia Coffee, the owner is or used to be on this forum.
Where are you staying?
Must do
Waimea Canyon
Polihale state park and beach
Food
Food is expensive, local items from the grocery stores are more reasonable (local meat, fish, some fruit, ect.). Some of the best food is at some of the local trucks and random locations. Opening times are just a suggestion, so have back ups.
In Koloa - Taco Libre is great, Big Monster has some good suishi. The Italian restraunt and Porky’s is also good.
In Poipu - Get some reservations at sunset at “The Beach House” food is hit or miss but the view is famous. At the end of March, the sunset is a little too far to the north compared to other times of the year, but still a great view.
Keiko’s Paridise is my wife’s favorite. Easy to get into and reasonable prices for the quality.
Make sure you get a Puka dog and lemonaide. Pinnapple with Hot sauce is the ticket.
Northside -
There are several Chicken in a Barrel, good bbq
Several good food places in Kappa
If you want to go to a Luau, the Grand Hyatt probably has the best food and experience. Every Tuesday evening.
Hiking
Māhā’ulepū Heritage Trail - In Poipu easy and good morning hike. Take it all the way Makauwahi Cave Reserve if you want to and see one of the caldera that is visible on the island. Best sunrise on the island
Waimea Canyon - there are a multitude of hikes in this area. Take the road all the way to the end and hike to the top. You can see both sides of the island at once at the top. Many different levels of hiking here and distances. Easy to very hard. The view down the Na Pali coast is amazing on a clear day.
Wai Koa Loop Trail - easy hike in an old plantation. To get to the old plantation grinding is several miles to and from. Beautiful through some of the plantation and tree areas. Did a hike with the Human Society for my wife to get contact with a dog, if that is your thing. It may rain..
Kalalau Trail - the short 2 mile hike to the river and beach, if you can get across is amazing. Not easy depending on conditions. If wet, walking poles are helpful. Do the whole thing if your feeling adventurous and willing to die.
There are many others around, even some water fall hikes near Kappa
Turtles
Should be able to see sea turtles every morning at Poipu beach
Beaches
Polihale - Best sunset I have every seen. Need 4 wheel drive vehicle to get to the end of the road. If you go during the week, this is very much a locals only beach. You will be secluded with very few people on the several mile beach. Might be one of my favorite locations on the earth with the back ground of the Na Pali coast behind you.
Kauapea or Secret beach - about a 3/4 mile trek through the woods to get the beach. Calm and very few kids at this beach. Don’t get a parking ticket…
Poipu - very busy on the weekends. The fact that you are going during springbreak time frames I would expect it to be busy every day. Good people watching beach if you are into that.
Māhā’ulepū beach and the proceeding beach to the east - just awesome because no one is there. No place to park and you need to walk from the Hyatt trailhead or Makauwahi Cave parking area.
Water adventures
There are several charters to the northside, both from the west and east side of the island. You can get some fishing charters too if you want. The biggest thing is do not expect smooth water. The water around Kauai can average 7-10 foot waves.