My favorite museum gift shop is the New York Transit Museum Gift Shop. They have some of the coolest things, like subway map shower curtains.
I'd never been to the museum itself until last week, when I finally ventured there with Mr. O'Kitten, his brother and his brother's wife, i.e., the O'Kitten bro- and sis-in-law. Located in an actual 1936 subway station, the Transit Museum is pretty big, with both changing exhibits and permanent displays, including at least one of every subway car ever in use. And at just $5 admission ($3 if you're under 17 or over 62), it seems to me every bit the bargain.
By the way, in the recent Straphangers State of the Subway report card, our line (the L) got top marks. Yay! But the cars look a little different now.