I have recently made a PCB of Arduino Countdown from http://hacks.ayars.org/2011/08/how-long-until-christmas.html this PCB have a lot of I/O pin headers that you can connect a lot of sensors, or others …

All informations, and code are available here:
http://hacks.ayars.org/2011/08/how-long-until-christmas.html

Add digital temperature sensor (DS1820) and clock, code is available (need to run under Arduino IDE 0.22)

The PCB was drawn under ARES (PROTEUS suite) and is available here