They played siblings Greg, Peter and Cindy on the classic family sitcom.
USA Today said they participated in four packed weekend shows at Kings Island in Mason, recounting their experiences on the series, answering fans' questions and screening some famous clips.
"I think everybody, at some point in their life, wanted to be a Brady," the newspaper quoted Williamson as saying. "Their whole family seemed so perfect. They were so nice. I think it resonates with the child in all of us to want to be Brady."