Tonlé Bati is the collective name for a pair of old Angkorian-era temples, Ta Prohm and Yeay Peau, and a popular lakeside picnic area. It's worth a detour if you are on the way from the capital to Phnom Tamao and Phnom Chisor.
You can eat at one of many picnic restaurants set on stilts over the water (the local delicacy is frog legs!) and hire an inner tube to float around the lake for 2000r. Just avoid Tonlé Bati at weekends, when it's mobbed with locals. Renting a motorbike or car from Phnom Penh is the easiest way to get here.
