My kids are about that age.
lego - so many spinoffs you can do from this. Challenges. Charades.
books - mine will read for hours. You could work your way through a book and then watch the movie. I mention Harry Potter a lot but my crew love this series. Percy Jackson, as well. 39 clues. Reading challenge. Read x amount of pages/books/hours and reward with.... ???
Old video camera? Create a movie. Or create a play for the rest of the littles.
Baking. They can choose/make lunch or snack a few times over the summer.
Sensory type stuff. Make goop, ooblek, etc.
Writing stuff/journals/notebooks. One of mine loves to write.
Drawing/sketching. I have 2 artists.
Art projects. I try to come up with a few special things for them to do. This is a good thing to have them do during a rest time. The artful parent (I think this is the name of the book) has some great ideas that my kids have enjoyed.
Carnival. They could set up a carnival with games/activities for the littles and/or themselves.
Gardening. A special little plot or container for them to tend to over the summer.
Make a fairy garden. There's a post on this.
I might set up a station or two outside that would be geared towards them. Mine like to do experiments at the mud kitchen (baking soda, vinegar, flour, food colouring, lots of utensils). It can get really messy but it keeps mine occupied for hours.
Honestly, I have a general inventory of ideas but mostly let them use their imaginations. The first 2 weeks of summer I plan nothing special. It gives mine time to chill out from the school routine and find their groove. If there is too much complaining I will give them a few choices or put them to work.
I don't like to 'do' too much for them, otherwise they become too dependent on me.