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.