San Diego State Historic Park is a free, open space with lots of commercial
buildings and some authentically restored structures sandwiched in-between.
 People often just wander and shop and eat . . . .  There was one 1820s adobe
that charged an admission and many visitors were turned off by the idea of
having to pay the $2 for the self-guided tour.  Last year they scrapped the
admission charge and I've been told that 1) attendance increased and 2)
donations increased.

You can always put a "suggested donations" sign up.