how about a barometer?  they're fun for the kids and they require you
purchase the Audobon Society Field Guide to the Atmosphere.  plus, they're
way more useful than a compass (at least in a car, they are).  though i
doubt VDO makes one to fit in a gauge cluster, or makes one at all for that
matter.

silly gauge suggestions aside, they're pretty useful for extended camping
trips, if you're a weather geek and/or sailed before.

jason