The best answer is to have spent your summer productively such as working as an intern. Even if you did only one intern, the interviewer will like that. The worst answer is to say you didn't do anything. Or say you worked in the supermarket to get some money so you can party at night. So think of something productive, and if you don't have an intern experience, you can say that you spent your summer learning something new.
Although I said don't mention useless work, it is ok if it is for tuition. It shows financial responsibility. Finally, taking some computer classes for the summer is a great way to prepare for work life and it was a summer well spent. Make sure you have an answer that shows productivity.