Bangkok in 8 hours

I'm on my way to Ko Samui and will be in Bangkok from 8AM until my plane leaves at 4PM. Is there enough time to do something in Bangkok? What should I see? I'm not really into shopping malls, but I was thinking of visiting a market or maybe even a temple. What is a "cannot miss" site in Bangkok? Also, how long would it take to go into Bangkok from the airport and how much would it cost? Is it hard to find a taxi or scooter to go back to the airport? Any advice would be much appreciated.