Something fun to do in the area is to rent a bicycle by the hour and ride along the paved South Coast Bicycle Trail which stretches over 20 miles along the L.A. Coast from Malibu south to Torrance Beach. Roughly 1/2 day down the coast in the middle of L.A. is Venice and Marina del Rey. If you rent a bike in Venice, go south on the trail, then follow the signs down Washington Boulevard and turn into the Marina del Rey area. This is a beautiful harborfront area, with over 5,000 boats, and lots of restaurants on the water. If you continue south from there, past Fisherman's Village, you'll rejoin the beach path and end up along the beautiful beaches of Playa del Rey. Pack a picnic, and bring along some water.
If you start in Venice, but head north, you'll arrive shortly in the fun beach town of Santa Monica. Look for the Santa Monica Pier to help you with your navigation, which is only a couple of miles north of Venice. Have fun!