For beaches, you have visited the best area, you must be aware of that. You can take a plane frpm BKK to Trang Provence and visit the small beautiful islands out there, like Koh Hai. You can then go up to Phuket by boat, why not spend a few days first at Long beach at Koh Lanta.?

If you go back to Bkk, you could vist Ayuttaya, which is absolutely charming.

We are going every winter 5 weeks, and have done this last time. We also went to Cambodia, to visit Angkor. Spend 3 days there.

You can also back,to Thailand by the southern coast stay a bit in Cambodia and then come back by road and make a stop at Koh Chang., or any of the other beautiful islands there.

We are going this year to Sukothai after resting the first day at Phuket, then to Hua hin. But if you like beaches the above sugesions are better.