The packet that carries them into the next grade.
A good summer packet is bounded: a clear start, a clear finish, and a parent-friendly answer key. One packet per grade jump, sized so a family can actually finish it before August.
Summer Math Review BUNDLE Grades: 3rd - 5th Three grade-to-grade summer packets in one bundle, keys included. 39 pages $11.95 Answer keys included See it on TPT → 
Summer Math Review BUNDLE Grades: K - 2nd Summer packets for the K to 3rd stretch, bundled so nothing slips over break. 39 pages $13.37 Answer keys included See it on TPT → 
Summer Math Review Packet Grades: 5th The bridge packet for the biggest jump there is: 5th grade into middle school. 22 pages $4.95 Answer keys included See it on TPT →