1. What is a praise that you have about today's visit?
2. What is a wish that you have about today's visit (I wish you would have...)
Through those two simple questions, we garner a host of useful suggestions and have a good gauge of what visitors are enjoying as well as what needs to be tweaked.
I use the same questions to evaluate my staff retreats, workshops and just about any event I conduct. Maybe the same two questions will engender some good feedback for you, too.
-- beth triplett