If you like seafood, while you're in Myrtle Beach, make a point to eat at Benjamins or Bennetts Calabash restuarant. In Savannah, you're gonna have to get to Paula Deans early in the morning to get a time to return for dinner.
Hmmmm! Let's see...what to do? Do you like shopping for tourist trinkets?...golf? ...walking on the beach? ... sunbathing on the beach? ...eating seafood? ...drinking in bars? ... repeat any of the above? ...waterparks? ...and our newest addition, the sky wheel, a huge ferris wheel on the ocean front that is really pretty to look at...blue and white. Enclosed boxes...$12 I heard.
Wilmington, NC is 50 miles to the north. It is an old seaport town and worth exploring. Ride up to Southport, NC and take the ferry. Cost is a couple of bucks. Then, it's a short, 20 minute, ride to downtown.
Georgetown, SC is 35 miles south. Another old port city, though not as prominent. Front street is quaint and historical. Take one of the boat tours up the river to get acquainted with several of the huge plantations that grew rice and indigo in the 1700s.
Charleston is 95 miles to the south. Lots to see and do there, especially if you are interested in history. I won't mention Savannah, since you have made plans. I would, however, suggest seeing Beaufort, SC on your way to Savannah.