Warm socks even if it is summer. Air conditioning might be too cold, comfortable shoes that you can stand wearing every day.
Exercise street smarts. People have been known to steal things so don't bring valuables or a bunch of money. If you hVe a hobby bring the things you like to do. One woman I knew made bead jewelry and brought her stuff with her to do that.
Bring a notebook and pens or pencils. Photos of your family.
We couldn't have electronic devices including cellphones. We each had a locker that staff would open for us if we needed things and our phones were stored in that.
Again, with toiletries, everything has to be sealed in the original container. No mouthwash unless it's alcohol free. And depending on if you are doing detox and where you are detoxing try not to bring things with strings-sweats, duffle bags, etc. The hospital I did detox at didn't have a specific detox unit and I was housed in the psychiatric unit, co-mingled with the psych patients, some of whom had been suicidal.
Most of all, bring an open mind to the experience. So many people came in with bad attitudes and just really didn't want to be there. You sound like you are willing, just nervous. I think we all were if it's the first time. Good luck! I had a great experience where I did rehab.