Here's a handout I used in a recent class in cartooning: one way to generate ideas for single-panel gag cartoons is to jumble a person from the first column with a place from the second column together with a prop from the third column. The first page here is adopted from a page in Mischa Ricter and Harald Bakken's 1992 "The Cartoonist's Muse" which is one of the exceptionally few books on the market that addresses the specific topic of content i.e. coming up with jokes, which is crucial component of a career in cartooning. This as opposed to the usual "how to draw" stuff that the majority of cartooning/comic art manuals devote themselves to (another being Robert Mankoff's "The Naked Cartoonist").
Used in conjunction with "Def Gag" exercize and Cartoon Jams, this is a great as a way to roll off the rough edges on the "square wheel," get the creative juices flowing, and "bypass the bologna" by circumventing the inner critic through timed rounds of captioning and cartooning.
Here's a sample customized for the Alaskan market (I have a similar one for Maine also):
One isn't necessarily limited to mixing between three columns: oftentimes just two will do, or four or more can be fun. The principle use is to prompt an ever-widening series of non-sequiturs, like playing with a Jacobs Ladder of ideas. The fun really starts when you start to shuffle between both lists...