At a minimum you need to have your thesis, pen and paper and something to drink for the viva.
Your thesis is there to refer to. You don’t have to commit everything to memory. It’s your guide to years of work.
Pen and paper is for making notes. You might need to work something out, depending on your discipline, or might want to record a question or a thought. It’s good to have it with you.
Refreshments are not always provided. Take something to drink. You could be talking for some time, and as well as staying hydrated you can stealth-pause by taking a sip of water!
These are the absolute essentials. What else do you think might help you?