Indeed Koh Samet is a nice island and easy to get to from BKK. I wouldn't go on weekends if you are looking for a quiet peaceful stay (too many partygoers from BKK during the weekend)

Chiang Mai is very nice indeed (and totally different from Koh Samet, Hua Hin or Phuket) but again you need to fly (admittedly the airport in CM is just outside the city center so transfers to/from CNX is not time consuming). I wouldn't go to CM during February-April due to poor air quality from farmers burning the fields.