Motivation always begins with interest any month of the year. When school is not in session, think of projects students have an interest in exploring. For instance, if planning a vacation, print out a tourist guidebook, usually available online at no cost by the local Chamber of Commerce and have the student select things they are interested in seeing and doing using a highlighter - more than one child - use multiple colored highlighters (good for spouses too!), be sure to do the same for your own vacation preferences. Then have the student coordinate everyone's preferences according to duration of time needed, cost, distance, and by days of vacation.
Suggest a nature hike and picnic they can invite their friends to join, then ask them to obtain the directions, the size of the area, trails available, the time needed, and to plan the picnic items for as many people are attending.
Whatever activity you plan over the summer - put the cognitive work onto the student to solve; not only will it keep their skills sharp, but they may enjoy the activity more because they've invested their own time, effort, and have contributed their own opinions and choices!