Merging/downsizing RFID cards somehow
Two options:


Emulate the cards. It's possible to build a device that responds with different IDs. This shouldn't work for "secure" cards like contactless payment, but for simple access systems it will work.
Dissolve the cards. They're usually made of acrylic, so you can melt away the plastic leaving you with a chip and a very fragile coil of wire. These can then be incorporated into another object.