I check out a book (started as Nabokov, but I ran out of him after about the 12th library) and walk around to look at the amenities and then sit and read for a while. Some have coffee machines and I'll get a coffee. I like talking to the librarians. Several of them are excited to see my uniform and we chat about scheduling educational programs.

I work across the whole state (or at least the forested parts), so I usually schedule my lunch break around a library I'll be near and spend my break there. It's nice to use the bathroom indoors and have a read for a little while before heading back out!