1) Read books that interest you. Nonfiction and/or fiction are both great for keeping your metacognition skills at their peak.
2) Keep an eye out for new opportunities (jobs, activities, events, etc.) that will challenge you, and keep you focused.
3) Set some SMART (Specific Measurable, Attainable, Relevant, Timely) goals for the summer and upcoming year.
4) Relax and refuel so that when September roles around, you are ready to hit the ground running.