Give your child a page with math facts on it (like a timed test page). Set a time limit and assign a monetary value to each problem. At the end of the stated time, the child is paid for each correctly completed problem. This money is then used to buy lunch.
Set up a "cafeteria" with various lunch items: sandwiches, snacks, and special treats. Put a price tag on each item, for instance "$0.05 for a pickle" or "$1.98 for a bag of M&Ms." Adjust your prices according to how much money was given for each problem (you don't want them to go hungry or stuff themselves), what area of math you want to practice (counting by fives, making change, etc.), and what you would prefer they ate (healthy low, unhealthy high).
This is also a great way to teach opportunity costs (economics): If you buy the chips, you won't have enough to buy the raisins. Or you can't afford both the cookie and the candy bar so you will have to pick just one.
My children LOVED doing this. It was the only way they do math without complaining. (We only did it occasionally as I normally didn't let them have snacky-type food.)
(You could probably "pay" your kids for doing other types of work too, but we always used math worksheets.)
