The "summer slide" is when a child regresses -- and forgets some concepts that he or she has learned over the school year. While summer should be mostly about fun and relaxing, you should still try to do some skill work with your child over the summer.
See this article for some ideas
For some fun with sidewalk chalk, click on this link!
Sidewalk Chalk: Ten Fun Ideas
If you are crafty, you can make your own!