For comfort, book the hotel limo. I've done the same. I believe it's a high end BMW (7-series?).

For a little more comfort than a taxi but less cost than the hotel limo, there's the AOT limos. You can "order" one as you arrive at BKK. They're not really a limo but a Toyota Camry and the like.

There's tons to see and do in BKK, definitely enough for a day. I'd suggest sightseeing in the afternoon (Grand Palace, Wat Arun, Wat Pho, Chinatown, etc) and shopping at one of the many malls (Siam Paragon, Gaysorn, MBK, etc) for the evening.