The folks at Adafruit share cool DIY wearable and connected electronics projects on their site on a regular basis. One of their most recent projects involves building a Mailbox Notification Service. The tutorial shows how you can set up a system to get real time notifications on your phone when you have a new package.
As you can see in the above photo, this device is solar powered. You are going to need XBee adapters, 2 x XBee Pro Module ZB Series 2, a 2500 mAh battery, a plastic enclosure, and a few other parts. You can find the instructions and all the parts here.