One summer my friends and I read The Hobbit and the Lord of the Rings trilogy from start to finish. That was fun, not only to read the books (which are great) but to do them as a kind of summer-long book club.
Another summer I tried to read all of Sir Arthur Conan Doyle's Sherlock Holmes stories. I didn't get them all done before summer ended (I read the Hound of the Baskervilles and The Sign of the Four after school started in September) and I couldn't get my friends to sign on, so it missed that element too.
Most summers I just read whatever I can get my hands on!