I highly recommend taking a guided tour of Park Avenue Armory if you are interested in history or architecture. This is a stunning late 19th century building which was built as the home for the 7th regiment in the 1880s. It is now in the process of being restored room by room and is also used by artists and for theatrical performances in the enormous drill hall. Tours run usually every month - I did have to book several weeks in advance to find the next available date. The $25 charge was very reasonable - it was supposed to take 75 minutes but was closer to 90, perhaps because our group asked lots of questions! But the guide, Grace, was happy to answer them all and was very pleasant and knowledgeable. We saw three rooms on the ground floor and two of the rooms on the first floor as well as the drill hall. The Veterans' Room with decoration by Tiffany and Stanford White was absolutely stunning. Most of the rooms have seating available so you can sit as you listen to the guide, and photographs are permitted.
One thing which could perhaps have been included was a bit more information about General Emmons Clark, who is represented in a huge portrait in the first room we visited. Grace had to look up who was the subject of the paintings in the room but it was only as we were leaving the room that I noticed there is actually a plaque to Emmons Clark under his portrait, thanking him for being instrumental in getting the Armory built, so I was surprised that he didn't feature more prominently. I presume the medals displayed by the plaque also belong to him.
Anyway, that is a minor nitpick - Grace was great and I really enjoyed it. I must say that the staff on the box office and the reception desk could learn something from her in how to talk to people - they were both very abrupt and surly, particularly the young man on reception who barked "TOUR?" at me as I approached the desk and exasperatedly told me that I couldn't use the bathroom until I had checked in. The email booking confirmation tells you to check in at the Mary Divver Room but there is no signage at the entrance to show you where this is - for information, it is immediately to your right when you go up the entrance steps (so you then don't have to go as far as the rude receptionist).