A fob device includes a portable housing below a size needed to accommodate an input/output component for providing access to a functionality of the fob device that would not otherwise be accessible. The fob device also includes a mobile processor, a wireless interface, a volatile memory, a rechargeable battery, a charging interface, and storage medium having instructions that when executed causes the fob to receive a pairing request from a modular extension device that can provide access to the full functionality of the fob device that would not otherwise be accessible, pair with the modular extension device, and provide interfaces to the modular extension device that are customized based on a form factor of the modular extension device.