A "Python" audio clip. Navigate to the following folder: /System/Library/Frameworks/Python.framework/Versions/2.3/lib/python2.3/test. Open the audiotest.au audio file you'll find there in QuickTime and play it back.
A hidden picture in Mail. In Finder, open your Applications folder. Select Mail, control-click (right-click) it and select Show Package Contents. Navigate down to Contents, and then Resources, and then open Senders.tiff by dragging it onto your Preview application icon.
Cool dates in history. In terminal, type open -e /usr/share/calendar/calendar.history
More after the jump.