At some point on your trip down this isthmus you'll end up in Phuket. About 530 miles south of Bangkok, Phuket is Thailand's main gateway to some 39 islands in the Andaman Sea. Although May kicks off the rainy season, you can still expect plenty of sunshine. Either way, plan to spend some time under the waves. Phuket is home to abundant brain corals, Moorish idol fish, even leopard sharks—inhabitants that make this area dive central station. Just east of Phuket on the Gulf of Thailand side, the best time to dive actually starts in May, when the water is clearest. Dive packages drop in price when the shoulder season rolls around, so much so you hardly need to be a diver to want to take advantage of these deals. For instance, book a five-night stay at the Andaman Seaview Hotel (+66.76.639.8111; www.andamanphuket.com) on Karon Beach with three days of diving out to places such as the Similan Islands for $346 when you go through Dive the World Thailand (+66.76.344.736; www.divetheworldthailand.com), a dive outfit based in Phuket. That's more than 30 percent off the high-season rate of $496.

If you're looking to blend your time off-gassing with one of those famous Thai massages, head over to Ko Samui in the gulf, an island that really began to take off after other areas were destroyed in December 2004. There, on Lamai Beach, you'll find an exquisite spa with 380-square-foot garden bungalows and about a one-and-a-half-mile stretch of sand so bright you'll need a welding mask to look at it. The best part, though? Not only does that 60-minute rubdown cost $15, but the shoulder season through June gets you a 15 percent discount on bungalows: $135 a night instead of $150 or more during the high season. Book the room through dive operator Dive the World Thailand and you can get screaming deals, like $486 for four nights and three days of diving, versus $542 during peak months.

And lastly, you can hit places like Ko Phi Phi from the Phuket side of the peninsula, too, which opens the door to scores of other deals. The Phi Phi Island Resort (+66.2.276.6056; www.ppisland.com) offers beachfront studios or luxury villas along a private beach. Once again, if you book through Dive the World Thailand, you can save a whopping 40 percent by going in the shoulder season. That means you'll get four nights and three days of diving for $654 as compared to $927 in the peak season. With prices like that, you've already saved nearly half of your airfare to come back again next year.