One of my biggest memories of childhood summers is all the time spent READING. We used to take weekly (sometimes, twice a week) trips to the local library. My brother and I were encouraged to take out all the books we could carry (and then some; sometimes our mother or dad wound up carrying some of them for us).
One of the reasons we went was that the library was air conditioned in a town where few places were (and our house really was not; my parents had a window unit in their room because my dad got migraines and sometimes being in a cool room was the only thing that helped him). Also, of course, my parents just generally encouraged any kind of intellectual pursuit we went after.
The library did a book club/reading club for schoolkids in the summer. I participated every year. Usually there was some kind of a set-up where you got a paper folder with a picture on it, and then you earned stickers or parts of the picture for every book read. I think one year there was a monster theme where you got monster body parts for books read, and then you could color them in and build your monster. I think also one year there might have been a horse theme? Where you got stickers of horses to put on a meadow scene for every book you read.
They also gave out McDonald's coupons for things like fries or shakes for every 25 or so books, but that was less of an attraction, as we didn't have a McDonald's right in town (at least, until I was in high school).
Really, for me, I think a lot of it was about the achievement and being able to claim I had read x number of books, and to see that add up with more and more stickers.
I continued to do the book club LONG after it ceased to be "cool" for my age group. And then, when I was in junior high and had aged out of the club, I worked as a volunteer for it, handing out stickers and coupons for kids who came in with the books they'd read. (Now that I think of it: it was entirely on the honor system and I suppose some people may have gamed the system to get more stickers or the coupons earlier...that would not have occurred to me to do, because for me it was all about being able to TRUTHFULLY say "I read 50 books this summer" or some such.)
I guess libraries still do book clubs...I see them advertised from time to time. I wonder how they've changed over the years, if they've become more "virtual," for example, having a webpage where you get your "stickers" rather than actual real stickers...