It is hard to know what to do your first time, but everything and everywhere is pretty good! We haven't yet been anywhere in thailand we didn't like, although we had some preferences. Our first time we took a 6 day tour to include CM, Mae Hong Sorn and Chiang Rai so we learned a lot. It's a pity you can't go north, because there is so much to see and do. My personal favourite is a 3 day stay at elephant camp in lampang, or cooking lessons. Next time we will be trying the Chiang Mai Nest resort. Mae Hong Song is also fun, as well as the golden triangle (we stayed at the anatara and it was spectacular.). Sukhothai we also felt was worth a visit (Orchid Hibiscus guest house is pretty much in the historical park, and is about £12 per night per room-run by a charming italian/thai couple).
But we also like the beaches we've tried, phuket and samui (prefered samui though). stayed in Central Karon village hotel, phuket which was glorious and peaceful- booked through latestaysdotcom and it worked fine. In samui we like the baan samui hotel on chaweng beach, again, peaceful and beachfront, but still on the main drag in case you want to be busy. We are fiftysomethings and like a mix of quiet and active. In samui do sign up for jungle canopy adventure, sort of like abseiling between treetop platforms,great fun and I felt safe. Took a 3 day excursion to river kwai jungle rafts hotel from bangkok which was good fun and added on a trip to tiger temple (sit with the tigers with the monks).

Hotels in bangkok are so varied and we have only used a few. started in marriott river garden first trip- luxurious and good for recovering from the flight with a charming complimentary rice barge for transport into the skytrain etc. But the location paled quickly and next trip we moved into the patpong area- we like the Silome Serene hotel but I know it has a mixed press. This time we are booked into the bel-aire princess (deal from travel agent to good not to take) so we'll see. Whatever you decide it will be great- just relax! We weren't able to cope with making plans as we went along, so compromised by booking all flights and then booking hotels as we travelled. You have to balance your own stress levels with flexibility!