If you use an RFID card, you can embed it right into your phone so you don't have to carry it around. Electronics DIY website Adafruit shows us how to do it.The process requires you to dissolve the card in acetone so only the RFID antenna remains. When the card is dissolved you wrap the chip and antenna in a few layers of packing tape, fit it snuggly inside your phone, replace the back, and you're done. Adafruit's guide is for the iPhone, but you can likely use it for any smartphone you have. It could be handy for work passes, but it might also be a great way to open your door at home. Head over to Adafruit for the full instructions.