Krumme Lanke is a great Berlin lake as it's easily accessible from the city, located at the end of the U3 U-Bahn line. It can get fairly busy on a hot summer's day, but buy a lilo drift out to the middle of the lake for some serious relaxing.
Pick a spot under the trees on the waters edge to set up your lake spot. If you fancy a skinny-dip there is a nudist beach on one end of the lake. Don't forget to bring food and water to the lake. There is also a supermarket across from the station, but remember it will be closed on Sunday!
Take a nice stroll on the circuit track, which loops around the lake.