Make a set of posters, one that reads "Today", another that reads "Tomorrow" and the last that reads "Yesterday." Stick them up on a wall in the school yard as far apart as possible from each other.
Make sure all the kids know what each poster says. Explain to them that you are going to say something which you do now, did yesterday or are going to do tomorrow. Explain that when you say the sentence they must run to the poster with the word for the day when you do, will do or did the action. When they reach the poster, have them mime the action you named.
Start by including the word for the day they must run to in the sentence, for example "Yesterday I cleaned my teeth". After a few times, just use the verb in the correct tense and omit the day. So say "I will walk the dog," and the kids should run to the the "tomorrow" poster. They will figure out what each one means very quickly.
Teach the continuous tense by playing the "I'm busy game." Have all the kids stand in a ring. The first kid asks any other kid if she would like to go and and do something. The person the question is aimed at must respond by saying "I'm sorry, I am busy swimming/reading/dancing" or any other activity and they must mime the activity they say. It is then her turn to ask a question to someone else.