The weekly trip to the library is a given. Hopefully, this takes place all year. During the summer, libraries frequently put on special programs for children. It is also a great time for parents to create some new twists on the weekly library visit. Here is one idea: everybody gets books from the 500 section this week to read about science, or everybody gets biographies, or everybody selects fiction from the first half of the alphabet. The person who reads the most in this section gets to select next week's category.
If summer vacation involves taking a trip, children can use maps to mark the route or to check for alternate routes, determine miles, and hours of travel. If flying, children can compare flying time to the time that would be spent driving. They can also write letters for brochures from the Chamber of Commerce about places to see (this is a good idea even if the trip is to visit a relative that you visit every year). Your librarian can help you select biographies of people who are from that area, books that have the area as a setting for the story, and lead you to books on topics that will enhance your child's appreciation of the trip.
Summer is often a time that children go swimming and bike riding. Have your children keep a daily log of how far, how long, how many laps, and how many miles. they go each day or each week. If appropriate, let them determine averages. Use their everyday lives as an opportunity for them to practice what they have learned. Children will have a much stronger understanding of their school work if they find themselves using it in their daily lives.
If a child has struggled or has weak skills, my recommendation would be to have direct instruction during the summer. Determine with the classroom teacher if there is a summer program or if tutoring is available. Some school districts have special programs during the summer.