I've just attended oSC19 and they had pretty funny game. They put papers with different QR codes all over the biergarten where conference was. Idea was to find all of them and for each you open web, you need to answer question about openSUSE. One of them was "first" one - the one which registers you in system and then allows to continue game.
Once you find 10 and answer correctly, you go to registration desk to pick up prize (mug, backpack, t-shirt).
I think this is pretty nice idea to keep people walking around and solving quest to get a prize.
For events which are inside building this is probably not interesting though.
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