Subtle ways to mess with an Instagram account?

For context, I had a client where I handled their social media account. I worked with them for a year before they recently ended my contract with them without proper payments. However upon checking it looks like I still have access to their account. What are some ways I can mess with it without them noticing?

I don't want to destroy their page or anything but make it a little more difficult for them ojt of pettiness.