While looking for a way to get onto the mass storage part of my Etrex Legend HCx, I found this cool trick: getting the GPS to display the temperature. http://www.weethet.nl/english/gps_garmin_etrex_diagnostics.php
is where I found the info.
Can't say it's the most accurate because it's showing 28°C right now, which would be very warm if it were true. But I thought it was a cool hidden feature nonetheless.