I think you will find Vientiane Kitchen nice. Serves both thai and isanfood, has a small stage with dancers and background music. We visit it everytime we are in Bangkok.
Its located in Sukhumvit Soi 36 and you can easily walk there from Thong Lo skytrain station.
You walk in the Soi 36 and it looks like you got lost... but then you see the guirlandes... and walk in... you get greeted by a nice traditional "Sabai dee" imported from Laos... While you get in, you see this huge tree in the middle of the half-indoor, half outdoor restaurant... and a band is playing music from Laos... The waitresses (some are guys disguised in girls..katoeys ???) are dressed traditionnaly... you get seated either sitting on the floor, either on a regular table and you start discovering the spicy laotian food... Good good time guaranteed !!!
Favorite Dish: Try the salted river fish with spicy sauce and of course Kaow Niao (stick rice...).
Local beers are fresh too ;)
you can eat northern Thai style, sitting on the floor and listening to the relaxing music
Favorite Dish: Snakehead Fisch