I had a dog that used to visit a psychiatric in-patient treatment center two days a week for a year. I took her. We would hang out with whoever wanted to see the dog for about an hour or so. The staff sometimes would document about the interactions if they were relevant somehow to the patient's plan of treatment. It was very, very rewarding. All I needed was to be certified as a therapy dog team, be current on vaccines, show that my dog was tested for parasites, and build enough relationship with the staff that they trusted me to interact with their clients. I also had to be trained about HIPPA (sp?) and sign a document regarding my understanding of the patient's right to privacy.
The facility was secure, as in the patients were locked in. I was given a key-pass so I could enter as I liked. Sometimes, depending on the clients/staffing/locations of visits, I was given an alarm that I could press if I got in trouble.
All in all, it was an amazing, humbling, and wonderful experince. We only had about 3 incidents that I can remember where things got tough. Unfortunately, the dog I did this with died. The staff and long term patients showered me with cards and trinkets when she passed.
If this is what you are considering, let me know if I can help. PM me if you would like. It's a very worthwhile way to give of yourself.