As I have mentioned before, during summers I enrolled my kids in the local library summer reading program. They earned a prize and coupons and kept up their reading skills also. I only asked that they read for 30 minutes a day, when they were older, and one easy picture book when they were younger.
I found GeoSafari units at garage sales early in our homeschool journey. I then found that it was possible to make your own cards to play on the machine. Then I found a yahoo group that shared the cards they made with each other. Since I had the ability to make cards to review any information or subject I wanted, I had the kids do several cards a day during the summer, one of which had to be a math card. I utilized this machine throughout the year as well. (Unfortunately ours are put in a place I cannot remember. )
Several years later I found something called a laptop geosafari. It is smaller and came with a larger number of cards. They covered things from sequencing to reading, the basic skills.
Sometimes we would play math games for another way to keep up their math skills. My favorite is NumberQuest. But you can really turn any game into math practice. This link will give you some ideas to use a simple deck of cards, modifying the rules to the card game of war.
I hope this gives you some ideas. You can always add in some fun cooking in the kitchen; try a new recipe together. Remember to have fun with your kiddos this summer.
Until next time, God bless,