There are many excellent sources of puzzles available. Now discontinued, a column in the New York Times – Number Play was great. 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 for students. Note the TEDEd lesson video here.
Staying with The New York Times, remember that is the home of the excellent Set Game illustrated in the above image. How many sets can you find? Check the rules. A new puzzle is set every day.