What do you usually do when your bored during the summer...?

Registration for summer school was non-affordable without assistance of financial aid, so summer school was out the window. The majority of summer jobs and internships that I applied to already informed me that they finished their hiring for the summer. Hanging out with friends becomes boring overtime as we see each other almost on a daily basis. Do you have any suggestions on what I can do during the summer?

Everyone has told me to self improve myself by learning a technical skill, academic subject, and hobby. I've learned how to cook, learn a new foreign language, exercise on a daily basis, eat balanced and healthy meals. I tried volunteering at a animal shelter and hospital; yet, they laid me off because they said they needed other volunteers (high school & college students) to have the opportunity. Other than that, I don't feel very productive, still feel bored and becoming depressed by being unproductive.

I have money and my parents suggested me to travel out of the world, but I have nobody to travel with. Any ideas/suggestions/comments?