When someone comes to town for a visit and only have one trip to do, we take them on the River Kwai trip. When we personally want to get out of the city for a day, Khao Yai is the place to go. So if you only have one day, I would recommend the Kwai loop. However, the prices on the site seem a little high. If a group of you were to simply hire a van and driver for the day, you would pay what that website is charging for one person. We literally had a tour company "forget" to pick 5 of us up one morning back in June... we went to the hotel's taxi stand and hired a driver for the day for the cost the tour company was going to charge a single one of us.

Another Day trip (with the possibility of stopping to ride elephants) would be to visit the ancient capital of Ayutthaya. I would pre-book this as you need the confirmed reservations for the return via river/diner cruise.