NFC isnt available on many phones, i dont think its even on the latest iphone. Only one android phone supports it.
It should grow in the future though. Re it being really fancy - its the same technology that has been around for years and years (granted, not on phones), google(android) have gotten involved in payments (and a few others), it got a new name and at it has come back on the radar again.
Best way IMO opinion is simple.
The user has to put in the name of the pub or whatever. and the app gives back a code/barcode/QR jobby.
That pub has a list (paper,electronic) and ticks the code off as being used. If its not in the list or already used you tell the phone waver to GTF. <!-- s:) --><img src="{SMILIES_PATH}/icon_smile.gif" alt="

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The principal will be the same; a central list that provides the voucher codes. and a subset of that list at each site location.
The detail is in how you implament the list at the site. i.e the pub could have access to barcode scanners, NFC readers, etc. but not all would have the same.
... I feel like im in work... oh wait... I am.