How about book vouchers? They can be spent at independent book stores if people choose to support them. Most people have some kind of independent book store near them. And knowing the wikiholics who might win 'em, the money will eventually come back to improve Wikipedia as a fair number of people will use them to buy reliable sources and then edit articles on the topic.

 

If someone is likely not going to be able to spend them, we include a very small proviso saying something like "if the prize is not likely to be redeemable, Wikimedia UK may at the discretion of a member of staff or the board of trustees (delete/edit as politically appropriate this week) provide a similar prize of equivalent or near-equivalent value".

 

A hack day I went to a few years ago had a really amazing prize: two tickets to the British Museum's then current special exhibition. This is one of my favourite hack day prizes. It wasn't a big and expensive prize like some other hack days offer (iPads, foreign travel, significant amounts of cash etc.) but it stands out precisely because it prompted me to go to an exhibition that I wouldn't otherwise go to and learn about something I wouldn't otherwise take much of an interest in.