Subtract 1-, 2-, 3-, 4-, or 5-digit number from a 5-digit number
In other words, they never memorized their multiplication tables! Many times I have seen a student do poorly due to a weakness in basic math facts they should have learned in third grade.
Another important point I keep in mind is that I never want this regular math review time to take up and hour of class time. I want it to be quick but effective. This is not instructional time, but time for the students to review material they have already learned.
Ratios and proportions are likewise wonderful math lessons with plenty of interesting practical applications. If three pans of pizza, one kilo of spaghetti, two buckets of chicken can properly feed 20 hungry friends, then how much pizza, spaghetti and chicken does mom need to prepare for birthday party with 30 kids?