Most teachers plan on needing to review basic subject-area content during the first few weeks of school. Why? Because most students need to refresh their memory after spending their summers swimming, playing video games, and generally going everything they can to avoid anything remotely resembling school work.
Summer enrichment doesn't have to look like regular schoolwork. Anything that gets your kids thinking will be helpful. This could be as simple as visiting a museum, a library, or a state park. Many museums offer free admission days - you just might have to call and ask. Most libraries offer free summer reading programs with incentives like gift certificates or small toys.
Going camping this summer? Challenge your kids to find ten different types of bugs (no, they don't need to pick them up and take them home!). Or ten different types of plants (again, no touching necessary). Challenge your kids to find similarities and differences between these ten items.
For older kids, watching Neil DeGrasse Tyson's "Cosmos" series may get them thinking about physics and astronomy. Follow it up by spreading a blanket in your backyard and stargazing. Or examining rocks in your neighborhood and talking about where they came from, and how they got to their present location. Check local universities for kid-friendly events; these usually have great content for minimal cost. Many may even be free!
If you play a sport regularly (tennis, golf, soccer, etc), bring your kids along! Have them keep score while you play to strengthen math skills. Give them a brief lesson in how to play before or after your practice or match.
Encourage your younger kids to help you pair socks or put away silverware (make sure you remove any particularly sharp knives first). Star charts are a great incentive for younger kids.
Maintaining a nightly routine of reading before bed is a great way to give your kids some extra reading practice. Reading to your pre-K child at night helps to build a great foundation for those vital pre-literacy skills.
For students who are struggling in school, year after year, maintaining a weekly spelling list or doing 5-10 math problems a day can help keep their minds sharp over the summer. You should be able to find practice workbooks at any bookstore, including used bookstores. Engaging a tutor to visit your home once a week may also help your child excel in the coming school year.