Just a couple of thoughts:

Would it be possible to design your survey (graphically, that is) so
that it becomes a self-addressed mailer?  That way visitors would
have the option of taking the survey home with them, completing it at
their leisure, and mailing it back to you.  Even if only 10% of your
visitors actually sent the survey back, that's 10% more responses
than you would have had otherwise.  Of course, your survey would have
to weigh in at the 32 cent postal rate.

Back along the incentive line...another idea might be to offer a very
minimal sort of incentive, like a postcard from your museum.  There
could be a selection to choose from.