It is true that over the summer 1) you don't necessarily need to juggle extracurriculars, 2) the compressed term means you may not digest or retain the material as well, and 3) some courses may be made a bit easier to accommodate the faster pace. It is okay to take some summer courses, but they don't want to see too many prereqs taken over summer terms. Also they do want to see you doing productive activities (that is more than just taking classes) with your summers.