My DS5 has been out of school for two weeks, and I was starting to worry about him forgetting math facts over the summer, so just last night I dug out a third grade workbook that someone gave him as a gift months ago. I merely opened it and left it lying in his view in the family room, and by bedtime he had completed several pages and was whining because he wanted to stay up to do more. This morning he asked if he could do a few more problems before we took him to summer camp. So, I would suggest simply making materials available. That might be all it takes.
This is exactly how my DS6 is.
