Where is a good place to take my Girl Scouts troop on a field trip that would be free or very low cost?

I’m a Girl Scouts leader with Daisies, Brownies, Juniors, and one Ambassador and looking for trip ideas. We’ve already went to a bank, baseball game, roller derby bout, nursing home, and soup kitchen, and am looking for more ideas. We’re hoping to have at least one “special” every month.