If you have a Garmin eTrex Legend (and this probably works for the Vista as well), you can enable diagnostics mode by pressing and holding the click stick and turning the unit on. You should see a screen containing information similar to this:

Software Version       2.36
Bravo Version             7
Since Reset      0013:51:23
Time               21:13:48
Temperature              22
Frequency          16368350
XO Drift             ---.--
Signal                 1769
SNR                    0.00
Battery                2.63

After the text, there is a graphical display representing the unit's various buttons. Pressing a button will light up that button's representation on the display (much like calibrating a joystick). There is also a series of shaded rectangles displaying the 4 shades of grayscale the screen supports. By pressing the "Page" button twice, you can flip through a series of display diagnostics (on these screens, the clickstick can be used to adjust brightness and contrast).

Have fun, and if you find any more hidden features, I'd love to hear about them.