What are the best things to do in notice period?
Work. Even if you have a bad relationship with your employer, then it doesn’t mean that you need to leave a bad impression of yourself with your coworkers/clients - you do not know if you will work with any of them in the future.
A substantial part of your day to day, as your last day gets closer, should be maintaining a strong handover document, spreadsheet, table, board, or whatever, with all the things you were doing, who the proposed new owner for each item of work will be, where things are at, and have meetings / convos with them so that your product(s) is/are set up for success once you’re out.
Build relationships with everyone you aren’t going to spend as much time with anymore. Your network is your net worth.
How long is your notice period? In Europe the standard is 3 months, in US can be <4 weeks. The answer to your question depends. Generally it’s advisable to remain productive and work, as your employer/manager/team will remember this period as well. Those people (unless you really hated the job) will stay in your network even if you leave. You get to decide how they remember you.
Trainings: online and saved ones in your company that may not be available outside but useful for your next role
I would make a note of all the processes followed in the company, best practices, pain points and document it in my personal email/ update the resume with roles and responsibilities we forget things after we move to a new company and sometimes these come in handy when your looking to make changes in your new role or looking for a new job and preparing for interview. Nevertheless complete all the pending work and never burn bridges its a small world we never know whom we would meet or work with/ for in future.