I think it really depends on the child and what interest them.
A school year is a very demanding frame, and children in school get to learn so many things they are not interested in, I think that if you are looking to make the summer educational and not just an endless series of computer games, it has to be something that they actually enjoy.
For me it was a library card, I loved reading as a child and every morning I would go into the library, pick up a new book and read it all day then go again the day after.
I think that also volunteering can be a really educational thing - a lot of cities have plans to help clean beaches and park, some zoos have programs for children for cleaning cages and learning to know the animals, even taking the old lady's dog for a walk everyday can be a very educational and helpful summer plan.
Also, a lot of art and sciences museum offer summer and day camps - and you can find a great program to match your child's wants and needs.