You can go to the Safari World for one day by taxi, it costs 400bhat for one way. It's very good. And you can join the local tour to the Floating Market, it's really interesting but needs to wake up very early.
For the safari world if you have more people but within five, it's better to take a taxi and buy the tickets there. Otherwise if you join the local tours it will be more expensive. For the food, I recommended the Oishi Grand(Japanese buffet restaurant) in Discovery Center in Siam Area. Its very cheap and has a lot of food to eat and drinks. I also tried the Vertigo Grill & Moon Bar (61/F in the Banyan Tree hotel). The environment is very romantic but very expensive. Chatuchak is a must go in bangkok as you can buy a lot of things there but only opens in sat and sun. Floating market you can choose the Ayothaya or Damnoen Saduak , the fist one is new and the other is very famous. Hope it can help you.