I found Penny Ur �s - Grammar Practice Activities - A Practical Guide for Teachers helpful.
She suggests a number of activities which may be suitable for your class.
In brief, she suggests you could:
1. Hand out a sheet of well known international symbols that express permission or obligation.
You can then ask the students either orally or in writing t tell you what the symbols mean using can, can �t, must, mustn �t.
2. Choose a game that all students know. After putting some basic information on the board about the game invite the students to list its rules using modals such as: may,may not, can, can �t, must, mustn �t.
3. Ask students to offer advice to anyone coming to your school for the first time. The kinds of things they should and shouldn �t do or must and mustn �t do. (ie You must come to school on time.)
She has several other good ideas and explains them in more detail than I am doing.
Hope this helps.
Have a great day!
