According to a couple of Log articles I've seen, the Mississippi Gulf Coast is supposed to be the next Big Thing. Beaches aren't as nice, but you do get the casino-related amenities.
Closer to Walton County, well I think for Coastal Walton County, you had to be here 18 months ago to really jump on that train. Further afield, I still like Mexico Beach east of Tyndall. They've had significant price jumps there, but still aren't at SoWal levels, and I think the nearby St. Joe redevelopment of Port St. Joe will push prices up in Mexico Beach & Beacon Hill as well. Quiet area, and the beaches are still sugar sand in that area.
Other possibilities: new townhouse developments in inland areas (Destin north of US 98, Ft. Walton Beach, Niceville). They're not short term rental areas, but are going relatively fast because people realize it's just going to get more and more expensive to live anywhere close to the coast. I got talking to a random realtor last Christmas, and she was saying she was making $50,000 per unit flipping preconstruction townhouses in Niceville in late 2004.
Growth areas of the future: Mossy Head, DeFuniak Springs. Now that mid-level service sector employees are getting priced out of Panama City Beach and Crestview (and darn, it sounds weird to say people are getting priced out of Crestview) they're going to need housing somewhere else, and those are two logical spots. If you can get ahold of a decent-sized parcel up there that can be zoned to residential and has good utility access, you will be sitting on something very, very valuable in 5-10 years.