Here are the rewritten sentences using the correct words:

6. We went on a picnic in the forest yesterday, and we tried not to leave the trash behind.
We went on a picnic in the forest yesterday, and we **avoided** leaving the trash behind.

7. I said, "Why don't we join a clean-up day at the beach this Sunday?"
I **suggested** joining a clean-up day at the beach this Sunday.

8. It took Mia three hours to pack the necessary things for the hiking tour.
Mia **spent** three hours packing the necessary things for the hiking tour.

9. Jay said that he would think about studying law at university as his mom expected.
Jay said that he would **consider** studying law at university as his mom expected.

10. Tom and Oliver say that they didn't disturb the nesting bird in the national park.
Tom and Oliver **deny** disturbing the nesting bird in the national park.