Definitely make reservations for meals and make them early.
The Festival of Houses and Gardens is going on now and will end on April 16th. Go to the Historic Charleston website to see everything offered.
https://www.historiccharleston.org/
The walking tours are at your own pace. There are docents at each house that give a small talk to the group. They also have folks on the streets helping with directions. The Luncheon Lectures are usually good. In the past they have done a short walking tour afterwards for anyone interested.
Definitely go to Magnolia Plantation. The gardens will be fantastic in April. Drayton Hall is next door. The owners of Magnolia and Drayton Hall were father and son. There used to be a small discount on admission if you did both, but I don't think they do that anymore. They give a talk before the house tour at Drayton either in the gift shop or out in the yard under some nice shade trees. Drayton really is my favorite house. Middleton is also on the same road.
Cemeteries? I love them too. And when I spend the day in Charleston I usually park in a garage on Cumberland and walk through the Circular Church's cemetery across the street. The Huguenot Church has tours during the week I think. Or if you attend services there on Sunday, someone from the church gives a small talk after church is over.
The Charleston museum is good, but if you are looking for something that takes less time, go to the SC Historical Society Museum. It is in the Fireproof building on the corner of Meeting and Chalmers. Website here https://schistory.org/
Go to Fort Moultrie on Sullivan's Island as others have mentioned. It was where a little band of Patriots in an unfinished palmetto log fort were victorious against an attack by the British navy on June 28, 1776. My 4X G-grandfather was one of those Patriots!
The last replica of the palmetto fort got washed away by Hugo, but they have a model of it in the visitors center.
Wear your good walking shoes for the historic district. You'll be parking the car and walking everywhere. Sidewalks can be uneven.
Most of all, have fun in Charleston and eat lots of low-country seafood.