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".
As always, I think if you read through
Bangkok (Thailand, Southeast Asia) | Frommers.com and
Bangkok travel guide - Wikitravel you should get a good idea as to what to expect and what is recommended.