For summer, we school year round, so they don't really get a break....I have no idea about losing retention or whatever... they never have because they learn sometime every day, even if we don't do our curriculum.
I don't structure their days. I send them outside if I don't want them having screen time. We do chores, school, chores, eating occasionally, playing games, playing outside, tv, video games, ipad/computer time, more chores
Not all of it every day necessarily, though sometimes they do. We have sports, AWANA, church, library, park/playdates... Nothing is actually scheduled except for the things we do with groups (AWANA, church, library, sports). Our meal times aren't even scheduled, haha!
We take the vast majority of our time off in the spring and fall. In the fall, we do school 3-4 days per week, and sometimes those aren't full days. In the spring we'll take a lot of time where they just play outside nearly all day long. I'll occasionally take school stuff out there, and most days we eat at least one meal outside.
My kids rarely ever actually get bored, but when they start in on the "what can we DOOOOO???" I tell them to either find something to do or I"ll give them something, but if they choose for me to give them something to do, they HAVE to do what i tell them... no complaining, arguing, whining, or whatever. They know this means any number of things (I'm actually going to start a 'boredom jar' when I get the shelves put up in our school room) and only truly ask for me to give them something when they are actually bored.