Not sure if you're open to non secular options but one we tried was gather round. It would be a fun summer school one because you can choose the topics and everyone does their thing together, hence the name.
If you're looking for something more structured and free - we did easy peasy. It also has a religious tone but unlike gather round you can remove quite a bit of it if you want. Easy peasy let's you essentially build the classes you want your kids to take... So you could do a language class, and math for one kid and then the other could do typing and language arts. I have nothing negative to say about either they helped us transition in our life to a new school which I'm appreciative of.
100%.... I closed the site when I was to stupid to figure out the example