experience as far as a place to stay (I get what the company will pay for which is usually not on the beach). Though if I were to choose a place, I'd probably look at the Henderson Park Inn as my hotel of choice (though it does look quite pricy).
As far as places to eat, there are plenty of great seafood restaurants around. The couple of places that I have enjoyed are:
McGuires, which is an Irish Pub and Steakhouse. Really fun atmosphere and a really good steak.
The Back Pork: This restaurant is right on the beach with patio seating overlooking the ocean. Also has really good seafood.
Stewby's Seafood Shanty: This is one of those small, order at the window, sit on a picnic table type places, and it's actually located in Ft. Walton Beach and not Destin, but it had some really good seafood (I particularly enjoyed the shrimp myself).
I know other people who have enjoyed other restaurants. I know there is a nice restaurant at Henderson Park Inn that looks really good (though again pricy - but for an Anniversary Dinner may be just the right place).
And as far as activities are concerned - I can't really help you. I was there for work so I didn't have time for excursions or anything.