I love to kill two birds with one stone, and this activity fits that bill. I use this as a post-reading activity to include in a novel study or use with a short story. Choose a problem a character faced in the story. Discuss the problem and how the character solved it.
Then, have your students brainstorm other ways the problem could have been dealt with. This is where the lesson does double duty with a sneaky grammar lesson. You can teach modals of regret (could/should/would have done, etc.) without getting too personal with your students.
If your students are lower level, you may want to begin with a complete grammar lesson with scaffolded practice, such as worksheets, for the students to get some more focused practice.