An onboard buzzer would be a nice addition. Since the Pi/Bo dont have LCD screens it would be a good alternative to physically notify someone to an issue/error they need to look at.. For example; after a successful boot and beep once if using internet time and twice if using RTC; if year is less than 2010 chirp every 20s; or if network is not available, a flood/freeze has been detected, garage door is about to close so play a tone, etc..
edit* If your going to toss a temp sensor onboard you might look at the BMP180 and putting it on the i2c bus w/the RTC, I believe it is very accurate and has a barometer which might be more useful than humidity, also has a native kernel module and the watchdog daemon should work with it without much fuss. (reboot if temps exceed XXX)