Originally Posted by JVPhoto

As lovely as the F lounge is, the F-C w/ 12 hours isn't that great.
I would:
Arrive BKK and have them take you on the buggy to immigration and fast tracked to enter Thailand. Not sure which day of the week but traffic may not be THAT bad where you could spend 35-55 min to get to Silom area and pay (incl tolls) about 400 baht (~10 US$). You could take the train and then connect to the MRT/BTS but that will take about 60-75 min probably. I forgot what it cost maybe 1/4-3 the taxi.

With that said I would do something like:
Head to Virgin Active at Chong Nonsi BTS/Sathorn Square (near The W) buy a day pass for 700baht. Do a little workout which will freshen you up and kick your jet lag, you then also have the pool/hot tub/saunas there to use as well.

After that you could find some breakfast, head over to Siam Paragon/Central Zen and do the mall thing if you were so inclined. Not far to walk from that is Jim Thompson House where you can kill an hour on a mildly interesting tour.
Depending on your interests you could see a movie at Siam Paragom. Get one of the love seats in the normal theatre or be extra HiSo (fancy) and see one in the Bangkok Airways Blue Ribbon theatres.
Grab some food and in no rush make your way back to BKK. You'll still get to do your choice of 30 min foot/leg or back/shoulder massages.

If you're there on a weekend you could kill a good amount of time st Chatachuk Weekend Market as well.

Originally Posted by BinSabai

beside JVPhoto's advise I would like to clarify lounge access.
After arrival you have lounge access to F and you can leave your luggage there and go down town and return (through fast track immigration of TG in entrance A) to F lounge. However should you decide to go directly through immigration and down town, you will likely not have access to F lounge in the evening as you depart on J