We live within walking distance of a pedestrian bridge which passes over BART train tracks. When Jack's train obsession started we took daily walks to the bridge and watched the trains pass by. Then we realized that if we waved to the person driving the train, he or she often waved back, and sometimes even honked the horn. Well that was such a thrill we started waving to other people: MUNI train operators, street sweepers, and garbage men have all responded quite nicely to our waves. The MUNI drivers seem to be the most jazzed by waving, and they almost always honk.
We took our waving philosophy one step further and tried waving to people in cars waiting for us to cross the street at stop signs or traffic lights. This has been the most dramatic interaction of all. People stonefaced with boredom or nearly apoplectic at the momentary delay we're causing transform completely when Jack waves to them. Their faces just light up as they wave back, and the rage seems to drain away, at least for a few seconds.
Waving at strangers is something you probably can't get away with as an adult, but if you enjoy the company of a toddler now and then (or all day everyday), I highly recommend it.