There are many excellent sources of puzzles available. One that non US readers may be unfamiliar with is a column in the New York Times – Number Play. A new puzzle is published every Monday. (The blog can be followed on Twitter: @NYTimesWordplay.)
At the time of writing the problem is 100 lockers which has a rather satisfying solution! This could be a good end of term activity with students.
Staying with The New York Times, remember that is the home of the excellent Set Game illustrated in the image above.How many sets can you find? Click ‘How to play’ in the menu on the left for the rules. A new puzzle is set every day.