Do you travel down I-75 from St Louis?
If so, best bet would be Clearwater Beach. Just go right at the fork of I-75 and the Florida Turnpike instead of left. When you get close to Tampa, follow the signs. When you're done at the beach, it's roughly 1 1/2 hours straight east to Orlando.
Many people, including my family, prefer the Gulf beaches to those on the east coast. Particularly if you have any children, they seem safer to me because you can go a long way out before the water gets very deep. Even without kids, the white sands of the Gulf beaches are much better for walking.
Sheila