MikMo, I think your suggestion sounds pretty cool! He could also monitor temperature and humidity inside and rainfall outside.
Still though, that would be a lot of I/O-lines not doing anything special.. Well, if he did your LCD-idea that would no longer be an annoyance.
Is it possible for one of the Arduinos to communicate with a computer while at the same time using XBee? If it is, he could log weather stats and put that on a website (mrtg graphs and all), along with perhaps a real-time updated XML-feed..
Who knows? I remember seeing something earlier today about interfacing the Arduino to X10 components. Perhaps you could even get your Arduino to control a few light or thermostats, or window blinds based on outside light conditions! You know.. just so no I/O-lines are going to waste. 
Controlling thermostats in your house would even make for some god-damned nice power savings, as you could turn them off/down at night, as well as shutting off lights after a certain hour!