My kids are still working away on their summer school workbooks. They are close to finishing, maybe another two weeks or so.
They just work away on them together, side by side. The motivation becomes their own when they remind each other that: they can go pick out a new toy, or a new pair of running shoes (what my 7-year old wants more than anything) when they finish them, and they race to their chairs and work vigorously.
That was the best idea - and the best $10 I've spent all year long.