Now, let's rewrite the sentences using the short forms of 'will not', which is 'won't'. Here are the completed sentences:
1. You (ты) will not rent a car. - You won't rent a car.
2. He will not rent a mountain bike. - He won't rent a mountain bike.
3. She will not make a sandcastle. - She won't make a sandcastle.
4. It will not swim. - It won't swim.
5. We will not sunbathe. - We won't sunbathe.
6. You (вы) will not dive. - You won't dive.
7. They will not sail on the lake. - They won't sail on the lake.