Summer Schoolwork

This will be my third (maybe fourth?) summer using these workbooks. I have one for each child. They don't cost much (under $15 on Amazon) and it gives my kids something to do each day to use their school skills.
The book is broken up into 3 months with 20 days in each month. My kids have to do 1 page a weekday. The pages can take anywhere from 5 min to 20 minutes depending on age. It's not a lot, but I think it helps them retain some of the things they might otherwise forget over summer. I know they go up to 7 into 8th grade because we bought one for this summer. I don't know if they go any higher than that.
I have found that the younger grades tend to be a bit easy. My son's K going into 1st grade book is quite easy for him. He's already completed 9 days (it's only day 4 of our summer) because he can do a page in a couple minutes.  One year I purchased the next one up for each of the kids. That worked well for the younger kids.
I grade the pages each night (there is an answer key - never fear algebra illiterate!) and they have to correct their mistakes from the previous day before doing the current day. That way they aren't just rushing through.

http://www.amazon.com/Summer-Bridge-Activities%C2%AE-Grades-Activities/dp/148381582X/ref=sr_1_2?ie=UTF8&qid=1432857371&sr=8-2&keywords=summer+bridge+workbooks