Hi Cocosf. This all depends what you are both into and also where you are staying. There are fireworks on the Chao Phraya river, I can't remember who sets them off (whether it is government or hotels).
I'm sure people will give you lots of good advice where to go, I would say this --- try to stick to a small number of places as the traffic build up can be quite substantial on NYE.
If you want to watch fireworks you should be nearer the river --- if you don't have a hotel, maybe ASIATIQUE might be a good place.
If you want to go party, a reasonably safe place is RCA.