When students see an expression such as "7x," they often find it difficult to accept that "x" could mean any number, even zero. Try to replace the "x" with something they do know, so that they can see that "7x" really just means 7 sets of some undetermined quantity. Present them with baskets of apples or bags of candy and lay out several identical items, such as three apples. Tell them that this is "3x." Now ask them to make "5x" or "9x." Let students answer worksheet questions using fruit or candy instead of writing it down.
Put students in small groups. Write out a monomial on the board. Start with something as simple as the constant 5 or the variable x. Ask the teams to come up with a list of different ways to create that monomial. At the beginning, you can allow the teams to work separately and for both to win if they get a certain, limited number of solutions. Later, you can turn it into a race to write correct solutions on the board. This game also gets harder as the monomials become more complex, e.g. 3x^7 or 2a^3.
Students can play this game as individuals or in teams. After the initial lesson on monomials, the teacher presents mathematical expressions on the chalkboard, on an overhead or on large cards that everyone can see. The only question for each expression is whether it is a monomial or not. Right answers earn points that can be exchanged however the teacher wishes.
Teachers can follow the familiar TV show "Who Wants To Be A Millionaire?" by selecting volunteers to answer a series of questions about monomials that receive increasingly large compensations. The questions can begin with identifying monomials and progress to solving equations that produce monomials as answers. Just like the show, the student can get a limited amount of help from friends. Instead of money, the teacher can award extra credit.