Dedicate even a mere 2 hours a week to studying a subject or two that they learned in school and you will be refreshing that education they worked so hard to earn in the school year. It doesn't have to be intense, and you don't have to make it painful; invest in a Wii learning game or board game that requires the use of brain power.
Who says you have to be in a classroom to learn? Take your kids on nature hikes or to a museum or science center; they will love it and will be earning some smarts without even knowing it.
Teach them to keep a journal over the summer break in order to strengthen writing and english skills. It also forces to them to do a little reflecting, and at the end of the summer, they have a book of all the fun things that they did over break.
Encourage reading. Strong reading skills are at the base of any good education, and if you teach your kids to like reading, the chances are higher that they may sail through school with ease. Working a reading regime into summer break will make for an easier transition back to school in the fall and maybe even foster some academic progress.