Learn a second language.
At your son's age my son still loved to cuddle up and be read to and discuss books. Sometimes he read to me to give me a break taking turns...but instead of it being academic time it was more "our special time together doing something we both enjoyed"
But I do know about trying to keep some of these kids mind engaged...or else we have major sibling problems....we were go go go go all the time.
Library programs, summer movie programs at the movie theater (low price but second run movies, we didn't care), parks and recs special events, stacks and stack of books from the library, scavenger hunts looking for books needed to be returned.
One summer we read a sponge bob book where he started a newspaper so my son wanted to write out own newspaper...I still have it....so stinking cute. Now that he is a big 15 year old we happen to be friends with the owner of the local small weekly paper...it would have been so fun to tour his business and learn from them and show them his paper when he was six. I love those spontaneous projects that they dream up.
Oh...Home Depot or lowes kid building projects where you go and build all sorts of stuff. I think at Home Depot you get an apron and pins for each project. My kids ate that stuff up as little kids.
