What you put into your internship is what you get. Many times I didn't have anything to do from my direct supervisor but I would go around asking several managers in my business unit if there was anything I could do for them. Towards the beginning, most would tell me no but over time I would get some assignments or side projects to work on. Once those were done I would repeat the process until I found meaningful work to do.
Several other interns would tell me they would sit around their desk bored if their supervisor didn't have anything for them to do. You can have more than 1 mentor, especially if your original one is super busy with meetings. So my biggest advice is to simply ask around for work. I'm thankful for the accounting manager I met over the summer, she's the main reason why I wasn't bored out of my mind during my internship.