I like Nong kai หนองคาย. Its a nice little town on the Mekong river. The people are friendly and it has a lot slower pace then Bangkok. There are some cool things to see there. Sala Keoku ศาลาแก้วกู่ is a must see. Its a sculpture park. Also you can cross the Mekong into Lao P.D.R. at the Thai-Lao Friendship Bridge and there are a lot of Temples in this town. The Thasadey market is a nice place to visit too.
I have been here twice before. This time I am here for the Naga Fireball event. At the end of Buddhist lent (The full moon of the eleventh lunar month) along a 50 k length of the Mekong river; fireballs shoot out of the river. Its a big thing in Thailand. I first found out about it around 12 years ago and have been wanting to see it ever since. OK, if you don't know what it is. Here is a link to explain it. Naga Fireballs.
I stayed at the Mut Mee guesthouse. The guest house is in a big garden and has a nice relaxing atmostphere. There are tables with little roofs over them and a kitchen so you can order food and drinks. Also, on the river there is a restaraunt, bar and a boat that goes out for a few hours at dinner time (you can eat on the boat too). I rested a few days there then went to Pompisai (a town down river) to watch the fireballs. I took Mut Mee's boat there and back.