Originally Posted by
transpac
Farther afield: Phimai Historical Park in Nakhon Ratchasima; Phanom Rung historical park in Buriram; as mentioned: Sukhothai Historical Park/Si Satchanalai historical park .... particularly Phrae and Nan.
I can second all of these.
Last week I visited Phrae for the first time and quite enjoyed walking around and seeing all the wonderful wood houses. Neither it nor Nan has much of a tourist infrastructure but that's what make them wonderful.