There are many things to do it all depends on what interests you.
The Ayutthaya by River Sun Cruise is very popular. You will see the ancient capital, temples, Royal Grand Palace and a wonderful cruise of the Chao Phraya river. Have a look at other suggestions on what to do from this website if this does not interest you. This is one I have been meaning to do but have not been allowing enough time in Bangkok.
If you had 2 days/nights then a trip to the Bridge on the River Kwai, in Kanjanaburi province, would be possible.
The night before visit some of the night markets in Bangkok. Patpong, probably the most well known one for other reasons, could be worth a visit. Not sure if the floating market would be open at night but it is 80 kms outside of Bangkok so may not be possible in a few hours.
Patpong is good for the comic value, but otherwise relatively seedy. Not dangerous seedy though (Entering bars could be dangerous to your wallet, but walking down the street isn't). The markets are dearer than other places. Instead if after a market, I'd have a look at the "Suan Lum Night Bazzar" (accessible via the Subway system). They have a decent beer garden too and a nice place to buy some food and beer and relax.
24 hours isn't too much time to do much else other than eat, drink, shop and see some of the close tourist sites.
I'd recommend catching one of the Chao Phraya Express Boats up the river for a look at some of the Wat's (temples) from the river and also see how people live (Maybe up to stop N15 or so). Don't catch the Tourist boat, but rather one of the normal flagged ferries. Watch boarding and disembarking as you could end up in the river -- I nearly did due to a shifting boat and someone jumping into my path!
The Grand Palace is an interesting visit, but I have only looked at it from the river.
There is a cluster of shopping centres (MBK/Siam Plaza etc) accessible from the Skytrain (National Stadium stop IIRC). They are good way to while away excess time and some items are pretty cheap, but I rarely buy anything.
Another thing that is a bit unusual is the "Snake Farm".
Is this your first time or have you already done the usual tourist sights? Assuming you have, the dinner cruie sounds good and shopping of course! I also never pass up the chance for a massage (straight) when in Thailand! For only 24 hours, you can choose between Pratunam, Sukhumvit and Patpong for shopping.
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