I just found this great list of free Summer Reading Programs offered by different companies through MoneySavingMom. The best from that list (IMO) is Sylvan's Book Adventure as it offers a whole online program with quizzes, prizes, and parent monitoring tools.
From there, I got curious and went to Sylvan's main website to see if they had any other free learning resources we could use over the summer ... and was happily surprised! They have 5 more resources, including a math activity booklet, "just a page per day" workbook, summer fun and learning guide and writing journal. The math booklet does not specify an age group, but after looking at the PDF, it seems to be geared towards jr. high/high school. Most of the problems say "Grades 9-12" or "Grades 6-12". The Page Per Day workbook, however, is split up by age group and looks great - a good mixture of math and language arts. The only problem is it's only 11 pages long - 8 if you take away the title page, sample calendar and blank calendar pages at the end. So ... that won't last the whole summer. But it does remind me of this other website I found a while ago that has some great workbook printables - for free as well. At Education.com, you can print off cute, colorful worksheets that are categorized by grade level and topic. Not all are free, but most are and they have a LOT. At least ... the last time I was looking through the site (probably almost a year ago now), there were. Let me know if it's changed! Since M seems to take after me in her academia successes/struggles - picks up on language/reading easily but has a harder time remembering math facts - I'm hoping she can pick up some speed this summer with a little extra effort.