Tonle Bati is a popular lake and picnic area with bamboo shacks built out over the water that you can rent out for eating and whiling away the day in a hammock. It's generally a weekend get-away spot for local people, which means it can be nice and quiet during the week. Locals swim there, but the water does not look really inviting. There are all kinds of food and drink stands that sell everything you need for a picnic along the lake.
Note that there are sometimes vendors that follow you if you arrive on weekends and try to get you to go to their place. It’s best to pass by them and find a spot on your own.
Always check prices beforehand on everything as they have been known to hand you an outrageously high bill when you depart.