Tonle Bati is an hour south of Phnom Penh. It consists of a small lake where there are a number of restaurants.
There is also a small temple complex. The temple was constructed is of a similar era to Angkor Wat. There is a small fee to enter the area and take some small notes to give to the old people.
The main temple is called Ta Prohm of Bati. There will probably be no other tourists there so and after the crowds at many places it is a very peaceful place. Enjoy a meal at one of the restaurants on the lake.