Teaching the modals (must, can, should and the negatives forms) can be fun but it can be also be a little challenging. Students often confuse when is appropriate, or indeed correct, to use the many different modals. The best way of trying to help them and ensure they use the word you have taught and ensure they know when to use them, is to get them to practice using them.
So, why not give them a board game? It's easy to set up and demo in class and gives you the chance to monitor and correct their use of the language.
Shoulda Woulda Coulda simply involves your students moving around the board, producing sentences that include the modal they have landed on. For example, if they land on "must" they could produce the sentence "You must take your passport on holiday" (not strictly true as you might be staying in the same country, thus the need for a passport is nil, but the idea is right).
It is also photocopiable, so quick, easy and cheap to print and duplicate.