Here are suggestions for each situation using modals of suggestion:

1. He sees a lot of trash in the school's playground. **You should pick it up.**

2. The water is running when she is brushing her teeth. **You ought to turn it off.**

3. He needs to buy some furniture for his new house. **He could go to a charity shop.**

4. They have a lot of books and toys that they no longer need. **They should donate them.**

5. She will move to a new house tomorrow and has a lot of belongings. **She must pack immediately.**