At Oakland’s Kinetic Arts, even seriously active seniors are unlikely to get washboard abs in 20 days. But you will have a hell of a lot of fun. Their exercise program is based on circus acts and many classes are tame enough for geezers. Instead of treadmills and free weights they have trapezes, hula hoops, hammocks and rope tricks. A program of 12 classes runs $150 but you can also drop in.