Animal Subtraction is beautiful for all children learning equations like 7-3 = 4. While the bulk of your class learns subtraction, your top students will be engaged by tough problem solving. There are also puzzle-sheets which require harder subtraction problems like 35 - 21 = 14.
The 45 minute time is what I've listed for the time, but you could easily do animal subtraction once a week for a month. Many students benefit from creating their own puzzles, and others will enjoy looking for general patterns in the puzzles they solve.
Animal Subtraction comes from a problem posed in 1967.