The best exhibit at the National Cryptologic Museum is the WWII-era ENIGMA (3-rotor, but still!) machine that is on display.

In the open.

With instructions on how to encode/decode messages using it.

And little slips of paper and tiny golf pencils right there encouraging you to use it.

This and the National Electronics Museum (colocated with the System Source Computer Museum [which also houses most of the DigiBarn collection]) about an hour north have more hands-on exhibits of actual vintage technology than practically every other museum in the country combined.