What if in the notice period we don't do any work, what kind of actions they can take

Nothing will happen. Just make sure you complete your KT. You are only responsible for KT in your notice period. They can't extend your np if you have not taken any leave.

1. NP being extended
2. Resignation Withdrawal
3. Disciplinary Action by HR
4. Release Letter delay and bad character certificate

To list a few actions!