Gulf Islands National Seashore is a gorgeous coastal park in the barrier islands of southern Mississippi. It's really one of the most amazing places in the state: gorgeous white sand, palm trees, hidden and uninhabited islands, nature reserves, and 200-year-old Spanish forts. Like I said, it's one of the most amazing places in Mississippi and should be at the top of your list if you're visiting Mississippi (or the south in general) in the spring and summertime.
One thing you should defiantly check out are the boat rides to West Ship Island. West Ship Island is an uninhabited island off the coast and the beaches are just spectacular. White beaches, clear water, and home to Fort Massachusetts, a 19th-century fort whose ruins you can explore today. The best part is that it's usually not that crowded so you can have your own plot of sand and sun to yourself.