Ye Gods, Mouse, who dropped this on you with such short notice? I would need longer than 3 days to sort something like this. What I would do is speak to the cafe and ask them what they would like you to do first, always assuming they have the odd idea and are not expecting you to suddenly burst forth with something spectacular!
The thought I have is to think of a theme relative to the cafe, eg chocolate, tea, coffee, cake etc find as many books, including kids and cook books relating to them and when a customer comes in ask them to choose a book relative to that which they have ordered, give a card with it and ask for a quick review. Pin all replies on the wall and let everyone peruse them and possibly argue. Someone may review a book in the morning and another person might heartily disagree in the afternoon.
You could make up simple quiz for each table with memorable opening lines or quotes from well known books, anagrams of titles, you know the sort of thing. In the mean time you can act as master of ceremonies, going from table to table chatting, joshing, discussing etc. maybe dressed as a book worm? I do hope the establishment is keeping you in sustenance and beverages, I think it is the least they should do!!
