Kids of all ages enjoy manipulating their world and moving things around. By using physical objects that students have to group and count, you can teach addition, subtraction and multiplication. Give students a pile of pennies, blocks or other objects they can move around. Have them set up word problems or see how many different ways they can group them. This allows students to visualize and feel how the numbers are interacting rather than just trusting that they do what you say they do.
In order to teach something to someone else you have to be able to understand it yourself. Have students draw, sing or act out different math concepts to teach to other students. This forces them to thoroughly understand how something works so that they can explain it to someone else, and it gets other areas of their brains engaged with creative expressions. Students can work in groups to help each other understand things and present what they did to the rest of the class.
Numbers tend to be more fun when they actually mean something. Money is something that most children understand at least on some level. Have groups of children work together to create and run an imaginary store. Give everyone five dollars of play money and let them go around and buy imaginary goods from other students. Make sure that each group keeps a log of what they sold and for how much so they can check the amount of money they have with how much they should have at the end of the game. This type of activity is especially good for addition and subtraction and even decimals.
Students are used to having to take tests, so let them make a test as a good way to get them involved and thinking in a new manner. Tell students that they need to make a certain number of specific types of problems, and have each student put together word problems. Assign groups to put their problems together in a test, and have them exchange their tests with other groups so they can each do each other's problems. Being on the side of the teacher makes it easier for students to look at the number problems from a new perspective.