I live in Thailand, although I haven't had much oppurtunity to travel.
However, if you want to go to a nice beach town, you should try Hua Hin. It's cheaper than the islands, less commercial than Phuket and less sleazy than Pattaya (unless sleaze is what you want, of course-then Pattaya is perfect). Not only does it have a
great beach, but there is also a Thai boxing garden there where you can take muay thai lessons and get ringside seats for fights dirt cheap. You can also take elephant treks into the jungle and there's a huge national park just outside the city that makes
for a pleasant day trip.
Another good spot is Kanchanaburri. You can get a fan room at a guest house for about 20 RMB a night. The famous kwai river bridge is there, and it's totally free to visit. Also, about 40 minutes out of town there is a temple where the monks raise orphaned
Burmese tigers. If you bring a camera, you can get your picture taken petting the tigers. Most of the travel agencies also organize rafting trips down the river.
If you plan on seeing both Chiang Mai and Phuket in the same trip, you'd better fly. They are pretty far apart, and internal transportation in Thailand is bloody slow. The trains only go about 40 km per hour and most cities don't even train stations.