if most people have Fitbits you could set up a weekly work challenge for most steps.
I think I've heard about apps that will link in to whatever you use, but tbh the ones I've looked at are generally not well reviewed...
you could reward the winner with a £5 Starbucks voucher
I wouldn't be overly upset if there was no budget and I was fronting a fiver a week It's not a bad idea. (Or maybe fortnightly, line it up with our Sprints) Again, I guess with the team being international I might need to do an Amazon voucher, or a selection to choose from perhaps.