When you are glum...
What perks your spirit?
I enjoy confusing random strangers. For example; when you pass a random stranger that will casually inquire how you are, reply with "Horrible, thanks for reminding me."
Or, another example, say it is a warm day and somebody is all overjoyed to have a bottle of water, randomly run up and smack it out of there hands: For maximum effectiveness, ensure the cap is off. Laugh histarically about your robbing them of simple pleasure.
... I also enjoy sodoku.