I took geometry in ninth grade, and am assuming you're teaching about that level. I don't have any jokes to share, but my geometry teacher introduced me to logic puzzles, and I have enjoyed them ever since. If you're not familiar with them, this is the kind of puzzle you find in crossword/variety books (found on magazine racks) that gives a number of clues about a number of items in a scenario, and by carefully considering each clue, you can figure out each item's attributes. For example, Mary and John did not wear green on Tuesday. Doris wore red shoes the day after Ginger wore her white hat, etc.
When I was preparing to take the GRE, I was told that about 1/3 of the exam was this kind of logic puzzle...and they were right. (Although my graduate school didn't look at that part of the test, I did the whole thing, just to see what my score would be.)
Just for something a little different. :)