Interact with one stranger today
According to a 2004 study published in Science, commuting is associated with fewer positive emotions than any other common daily activity. And yet, for those of us who commute on our bikes or by transit we have so many opportunities to interact with strangers.
This article: http://www.nytimes.com/2014/04/26/opinion/sunday/hello-stranger.html talks about a number of studies showing that, while our instincts might be to avoid talking to or even looking at strangers, doing so makes us feel better. Interactions with strangers leave us feeling good for some time afterward.