Take your ATM card. Plenty of ATMs all over Bangkok. How can anyone answer your question? You can have a meal for 100 baht pp or 4000 baht pp. You can buy a tie for 50 baht or 5000 baht. You can take a boat ride for 2000 baht or the public river boats for 20 baht.

Be sure the taxi will use the meter (those taxis parked outside the hotel will gouge you.). 200 baht or less in taxi fare will get you to all the sites in Bangkok. Be sure to have 20 and 100 baht notes and not just a wad of 1000 baht notes.