Where do you live? Do you have a car? What kinds of things do you like to do?

If in Bangkok, then day trips to almost any nearby province can be interesting. Ko Kret, Samut Songkhram: temples, King Buddhalertla Naphalai Memorial Park, Rama II park/house, Bejarong Porcelein, etc. Chachaengsao and Chantaburi are also day/overnight destinations. Farther afield: Phimai Historical Park in Nakhon Ratchasima; Phanom Rung historical park in Buriram; as mentioned: Sukhothai Historical Park/Si Satchanalai historical park (BTW, I would recommend not going during Loy Khratong as it is over-run with extra lights, cables, fire-works cannons, vendors); Zodiac temples in the North; particularly Phrae and Nan.