Brooklyn's Atlantic Avenue Tunnel is the world's oldest subway tunnel and you can take a free guided tour of it. The tunnel, which was built in 1844, is opened up to the public about once a month. Reservations are required.
Click here for more info and to reserve your spot on the July tour. They fill up well in advance which is why we're giving you a heads up!
* Sorry all. It's been brought to my attention that the website is misleading. There is a $15 charge for adults and $10 for children. Still worth it in my opinion but not as good of a deal as it appeared! :)