For five years now I've been ordering these summer books to prepare my kids for their next grade level. The books consist of one page (front and back) per day for the child to complete and it focuses on math and language. You can also purchase additional books that focus on a particular subject like math, reading, SAT, etc. Also for parents the answers for each page is at the back of the book so it makes it easy for grading. I love these books because I am always afraid that my kids minds will go to mush during the summer and they would forget everything learnt during the school year. Plus your kids won't feel like they're giving up their entire day doing school work because like I mentioned already - its only one page to complete per day. During the summer I also have my kids read a book they enjoy and afterwards they have to write short sentences about the book.
If you are interested check out Summer Book Company here: