A friend gave me this idea - and so far it has worked marvelously, so I thought I'd share. (Hint: this may work best if your kids don't often get new toys, but it still might work well regardless.)
We bought workbooks for our two older kids, (and a sticker book for our 2-year old), and then we explained to them that if they completed their workbooks, we would let them chose any toy they want at the store. (We also said that we would get them additional workbooks if they completed them, and additional toys.)
Seriously, I have never seen my 7-year old son more determined to work diligently on completing a goal, (besides getting to the next level in a video game). During the last four days, he has competed 100 pages. Luckily for me the book is huge and has about 500 pages; but still, I thought it would last him longer than a month! Our daughter, with a typical second child approach to life, has been there right beside him, working determinedly.
It is so nice (this is an understatement) to see them sitting together at the table, working diligently with their workbooks, doing math and reading, or (in my daughter's case) cutting and gluing, or circling or tracing letters and practicing sounds of letters, etc.
Really, it is working almost too well. They work for a while, and then go out to play (in the rain) for what they call "recess." It's been really fun, and hopefully it will keep their minds active, and continue until school starts in the Fall.