I have few theater mementos, but not very many. I don’t even keep souvenirs from most of the shows that I have done myself. I didn’t even know how many shows I had done till someone counted them up for me several years ago (I haven’t kept count much better since). I have my Once Upon A Mattress program signed by Imogene Coca, King Donavon, and Edward Everett Horton; probably two or three Peter Pan Playbills signed by Sandy Duncan (I saw it four or five times), and two copies of the Playbill from the first preview of Titanic-The Musical which contains the songs that were later cut and also has the original Titanic poster design with the black background and the word Titanic as part of the design of the ship.
I have some other autographed photos (Angela Lansbury, Antonio Banderas, Streisand) and some other autographed CDs (including Night of the Hunter, BK, signed by Stephen Cole) and Playbills, but they are mostly tucked away in boxes.
I agree with LC that some of my best mementos are my memories of celebrities that I have met and talked to who were kind and generous with their time.