Apparently, there are quite a few of us who enjoy the idea of a coffee shop. I doubt I'll ever do it, but a year ago, I did a cost analysis spreadsheet of opening my own shop, but now a coffee cart seems like a fun side project. :)
They have such high margins on individual drinks (and people will totally buy a pastry if the coffee is free) that this shouldn't cost too much. Plus you can always make the puzzles harder, or have 2 or 3 easyish ones and then graduate people onto harder ones that they'll take a few days to solve.
You can imagine how to decorate the place - blackboards for walls, encourage the customers to use them to practise the spare puzzles lying around today, etc...