I went in voluntarily last year and it was much better than involuntary. I mainly went for a med change, and they just took away the Nuvigil that was making me manic. I just brought flip flops because I always wear them, and pj pants with just the elastic. Bring pants, because most hospitals will be cooler. I made the mistake of packing only shorts because it was about September, and I was cold most of the time. They let me bring my school books, but no pencil or pen.
Good luck to you!