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...