Hello, my lesson plan about modal verbs as speculation for intermediate levels is:
1 - brainstorm the word beauty and its connections, you can use all that springs in mind such as money, tummy tucks, dieting etc.
2 - bring some questions about cosmetic surgery and body modifiers and ask ss to discuss
3 - show some pictures of people apologizing (in many different situations) and ask them to speculate about what they may have/n't/ done and take this opportunity to introduce all the modal verbs in the past.
4 - Ask ss to think about things they regret (not) doing and let them free to talk about these things.
5 - Play Bon Jovi - Misunderstood (available here) song and use it to teach contractions, fluency pronunciation.
6 - Provide ss with a piece of paper and ask them to, individually, come up with sentences using modal verbs in the present or in the past. After that, ask them to challenge other students using those sentences, in a faster and natural way, and the listener needs to discover which modal verb is or even create a reply for the "modal" sentence.
Student 1 - I could've hurt you.
Student 2 - I knew the risk.
Student 1 - I could hurt you.
Student 2 - Then be careful.
hope it helps you.