I would add the Nancy Drew series - very much like the Hardy Boys, but more for girls. I read and enjoyed both as a child, and my grandchildren like both. The youngest, a girl, loves Nancy Drew - and has several computer games related to her.
I also loved Trixie Belden - it was a small series compared to Nancy Drew, but I bet I read each of them about 5 times. Don’t even know if they still publish it.