I personally think that Phuket and Koh Phi Phi are the most touristy places in Southern Thailand, the only difference being that Koh Phi Phi is fucking breathtakingly beautiful (place where "The Beach" was filmed). If you're looking for places in the south, I'd say hit up Koh Tao for some awesome scuba diving, Koh Phangan for the full moon party (yes, i realize the irony here if I call Phuket touristy), and maybe Railay for a good place to just chill, rock climb and generally just veg out.
Mind you, I'd rather backpack and stay in a dumpy little bungalow on the beach than lug around a wheel-suitcase and stay in hotels, so take my advice with a grain of salt if that doesn't sound like your idea of a vacation.
Cambodia and Vietnam are also very fucking top-notch in my books, but it eats up a lot of travel time.. not really feasible at just a couple of days for each country.