I have a table of brain teasers, challenging puzzles in my room. My students love these puzzles. I verify that they have completed their assignments before I allow them to visit the table on a daily basis. This has provided me with a hidden tool. Many students that would not ask me for assistance prior to having the puzzle table now want the opportunity to work the puzzles.
Extra credit is another way to challenge students.
I have found that even my A students love to complete extra credit. I always make sure my extra credit is challenging. This is also a good way to connect with the home. Many students inform me that their mom or dad assisted them with the assignment. What if questions usually provide instant challenges.