I LOVE mb. We go every year. We hit the beach during the day and at night there is Barefoot Landing, Broadway at the Beach (shops, rides, mini golf, IMAX), they have a new MBCommons shopping area but I haven't been there. There are go kart parks, water parks, a zillion mini-golf courses. We also love Dixie Stampede, there is a Medieval Times, as well as Alabama Theatre and the Carolina Opry. There are horseback riding places where you can ride on the beach. A children's museum (though I haven't been there). The Aquarium is decent too and there are a ton of other Ripley's attractions that I haven't been at in years. There are also oulet shopping centers and malls.
At the beach, you can parasail (which is pretty awesome), take banana boat rides, they have dolphin watch cruises. There is a zoo in Waccattee (though I would probably skip that. It's neat because you can feed all the animals like deer and bears and monkeys, but it's really sad because the conditions are not great for the animals. It almost looks more like a hoarding situation...).
There are some great restaurants too.