I used to live in Destin & Ft Walton - it's a great area. Not sure about the 4-wheeling - it's all sand & pines. If you get a pass for the Eglin range there are some fun trails for exploring.
October's a great time to go because you'll actually be able to traverse 98 through Destin at speeds faster than idle.
I'd HIGHLY recommend hanging out on the lower level of AJ's for the Destin Fishing Rodeo (runs through all of October). Lots of fond memories throwing back drinks on nice afternoon with the wife watching the charters lined up to take their turn at the scales. Ton of fun.
Also skip all the tourist places & go to Dewey's for dinner. Amazing food.