I currently use bPay. If you haven’t heard if it then it’s a sim-sized contactless card that you attach to your phone (lame) or have in a bracelet (awesome). You top it up like with Mondo and you can instantly see your spending in the phone and web apps.
This is what the card looks like:
Rather than buy an Apple Watch I simple sellotape it to my watch strap.
This means I can tap the bottom of the strap onto contactless readers, including on the Tube.
I’ve lost count of how many people I’ve freaked out with it. In fact one a girl ran after me to demand what kind of magic it was. It’s pretty damn handy!
So naturally I think you should look into making a companion sim-sized Mondo card which people can attach to their keyring, watch, whatever. Either make it sticky, or even better make a sticky plastic thing to attach to the watch which you slide the sim into.
What do you think of than then eh?