After they know the three and their differences, I can think of a game. You can have a poster on the board with past, present, and future, as well as some verbs with their correct tenses underneath the appropriate columns. You should cover up most of the verbs, so the students only see one of the verbs. You can turn this into any kind of game that you want, but the idea is the student has to guess the hidden verbs, and then bonus points for using them in a sentence. Hopefully this'll help you once you get past the "fun" of teaching them the 3 tenses.