Get dinner up at Calabash (NC/SC border town) one night.
Yes! There is an awesome seafood restaurant in Calabash. I can't for the life of me remember the name.
I grew up near MB. My opinion: Ripley's Aquarium is great fun. It's just a regular (for profit though) aquarium. The moving sidewalks through it are cool. If you have another at home skip it.
At Broadway at the Beach (same at Ripleys) is a really cool mini golf course with a fire breathing dragon. One of my favorites at MB. Actually all mini-golfs are amazing in MB! One our trip to WDW earlier this month we went to Winter Summerland and I was incredibly underwhelmed bu their putt-putt course. Definitely go to at least one of them (look for one where your once price lets you play all day).
BBQ (+ southern food) is amazing at Prossers in Murrells Inlet.
If you're already at Murrells Inlet and want to check out the quieter side check out Brookgreen Gardens. It's like a mini zoo with famous statues. Also, right across the street is Huntington Beach State Park. They have alligators, which the guests can actively see. There's also an old castle, Atalaya. Brook green has a higher price (Maybe $10ish per person) but the state park and Atalaya are just dollars per person.
I went to the Wax Museum as a kid. Probably depends on the age of your kids, but it's just wax statues of people. Might take being a little older. I remember it being pretty cheap.
Check out the Myrtle Beach Eye. I've never been on it but it's just like the London Eye but a view of the ocean. Apparently pretty great.
Alligator Adventure at Barefoot Landing is neat. I believe the admission is for two days. But be aware whatever the temp is inside there it's hotter (but without the sun) inside.
Captain's Plank in Cherry Grove was a really popular seafood buffet for my family. I'm allergic to shellfish, so when I was a kid we always stopped at this little pizza place (literally in the parking lot) and got me a personal pan pizza on the way in.
I think the winieries are fun. There's one in NMB Belle something. The best is at Ocean Isle (just past Calabash), not sure the name, but I'm sure there is only one.
That is what I have off the top of my head. Feel free to PM me if you had any questions. Like I said I grew up nearby and have been away with visits every 3-4 years for the last 15 years.