Summer is the ideal time in which to process what you learned during the school year. “Processing” sounds boring, but it means having fun and incorporating what you learned into your own life!
Project learning is a great activity for the summer, because you have time to let the project take on a life of its own. You could choose something that was interesting during the year but was left unfinished because of a busy schedule.
Of course, visiting museums is a great way to pick up threads of learning. Spend time in the exhibit discussing what you see. Ask if it relates back to anything they learned in school this year.
Creative writing is a great activity for that time in the day when its just too hot to be outside. Setting a time limit (even 10 minutes) keeps it manageable.
Summer camps are great too, but sometimes they can begin to feel like school, with so many activities and rules. Make sure they have time to merely be and think and experiment! Unstructured time can lead to interesting discoveries. This is the time to let kids be kids!