16h layover in BKK, any suggestions what to do?

Hi, on my way to Japan I'm having a long layover in Bangkok and I wanna use the time to maybe explore the city because I'll be bored out of my mind if I don't. My plane arrives at 6am local time and departs at 10 pm local time, so 16h.

Don't have huge expectations just generally get the city vibe, maybe one temple or so or other nice things to see. Never been to Thailand before. I see that there's a quick rail connection from the airport to the city centre. I think doing something that's not too far from those hop on points would be sensible, altho taxis seem to be very cheap there too.

How much THB should I exchange in advance?

Thank you!