Bigwaves, you don't say what level of cert you are arriving with (and there's nothing in your profile to suggest whether you are already a diver), but I'm guessing you have OW training--maybe AOW too. To carry out your goal of continuing your diving education while you're in Thailand, you could actually move around to sample different areas of the country, doing a cert or two at each place and just diving in between. You can find lots of good accommodations bargains on the Andaman side, especially, during the time you will be here. Here's what I would do if it were me:
If you are interested in wreck diving, you might start out in the Gulf of Thailand, at Pattaya. The diving in the Gulf is generally disappointing compared to the Andaman side of Thailand, but for wrecks, that is where you want to go. If you don't already have the AOW, you will need to do this before your wreck cert to get the deep dive intro from that course out of the way, and then do wreck diving as one of your three elective adventure dives for the AOW. That dive can count as the first dive of your wreck specialty, which can be your first specialty course as you move towards your MSD goal.
While you are in the Gulf area, you might want to head down to Tao to check out the two most famous sites there--Chumphon Pinnacle and Sail Rock. You might get lucky and see some big fish.
Now leave the Gulf and head towards the Andaman side. Even though the period you will be here is our low season, we actually dive year round. If you are willing to risk some rough seas while you are sailing, you can even find boats that go out to the Similans during low season (for diving they go to sheltered sites). Head for Phuket to pick up another specialty or two, such as Fish ID and Digital Underwater Photo (which work really nicely together). And then move on down to Phi Phi to do your Rescue Diver course. Phi Phi has some sheltered beaches and I think it's nice to do Rescue as shore dives--no rush to get all those exercises and scenarios done while trying to adhere to a boat schedule. You will also need your CPR/First Aid cert to become a rescue diver, so if you already have that, make sure to take your documentation along or you will have to do the EFR course, too.
Next you can make your way down to Koh Lanta and head out to dive Hin Daeng/Hin Muang if it is looking good at that time (there's been some mysterious soft coral damage recently, and the soft coral is the calling card for those sites). You can pick up another specialty that interests you, too.
Then you could keep moving south towards Satun to reach Koh Lipe, among the least developed dive destinations in Thailand. If you get down there in October, the ferries should just be starting back up again from Pak Bara (service shuts down from May to October). This is beach hut + little boat diving, and can be nice for a change of pace. Finish off your last specialty here. By now you should have the 50 required dives for the MSD cert, so it's just a matter of mailing in the paperwork.
