hotel in bkk for day-use
 

we have to spend 12 hours in bkk (waiting for connecting flight) and are looking for a good hotel there to spend the day

You are looking for Novotel Suvarnabhumi Airport Hotel. It's just outside the airport. There's currently shuttle service from terminal to hotel and back.

I spent a day there few weeks back.

If you want to head into town, and due to the limited time you have you may want to be quite central, I can recommend "Pathumwan Princess" which is part of the MBK complex in the city centre. Good for shopping, but the hotel has a decent spa and pool too. It's fairly priced too, which might make sense for day use. I'm a member of their gym (for the past 7 years) so can thoroughly recommend it.