I have done a couple of things. First and foremost, we play games. If you have the Math Card Games manual, pick a game that they have not yet played. Also, I also recommend to moms that they can pick a theme for the week. For example, play addition games for the week. Or, you can have a them for each day of the week. For example, Monday-Addition, Tuesday-Subtraction, Wednesday-Fractions, Thursday-Multiplication, Friday-Favorites.
Something else I have done is to save some of the Review pages or practice sheets and use them for the summer.
One other idea, I haven't actually done this, but always wanted to...
In each Level, there have been segments that my kids absolutely love to do. For example, in Level B all my boys loved the 'Banker' section - where they are learning to add in the thousands. In Level C, my kids loved the geometry section. I have always wanted to review those sections over the summer. The kids loved the lessons and they will always get more out of them the more the do it.
Anyway, I hope that gives you some ideas for summer learning!