Here's a useful attractive 2 page grammar worksheet on modal verbs (can, could, may, must should) with explanations, examples sentences and 3 different activities to go with it, where students are asked to fill in the blanks with the most suitable modal verb, choose the right modal verb for each situation and replace the underlined expressions by a modal verb starting as suggested.
This pdf document is also available in black and white version, with less clipart images to suit older students needs. The answer key is also included.