Just for something completely different: DIY Christmas!
While we’re using Arduinos and sensors to monitor the natural world, other people are using them to make awesome choreographed Christmas light displays. I have to admit, my husband got sucked in by GE’s iTwinkle Christmas Tree and thinks it’s pretty cool that he can control the color of its lights via Bluetooth. But we just have nothing on these creative people who make their whole yards flash and change color in time to Frozen. These guys are all kinds of awesome. :0)
They have a really helpful wiki and beginners guide to get you started: http://www.doityourselfchristmas.com/wiki/index.php?title=Beginners_Guide