I like the idea of a NFC device like this. I like the idea of it being all or nothing.
A 5BTC chip would show red for empty and green for full. It would also broadcast its public key and be willing to sign a challenge number to prove it really controls that key. That way you can check the balance of someone elses chip and confirm no trickery is going on.
It would also be able to sign a transaction of its full balance, but only if a switch or button is in the "spend" position.