What should I do if I don't get an internship?

You may consider following options:

• Join a volunteer group of some sort: 
  
  You will make new friends, have a great sense of accomplishment, stay active, and even help others! 
• Improve your writing skills:
  
  Being able to produce written documents in a clear and concise way is important. Lacking adequate communication skills can inhibit an engineer’s ability to get a job at some of the larger technology companies. Engineers are often expected to report on their work, not only to share with their colleagues, but also to add to the corporate memory.
  
  
  Have a look at this Coursera course!  OR Set up your own blog and start writing about the topics that interest you.Blogger.com or WordPress.com  
• Start a summer business:
  
  Start your own business instead of working for someone else. Besides accomplishing the main objective of making money over the summer, when you start your own business, you'll have the satisfaction of being your own boss and of creating something yourself. You will improve your communication skills and learn to manage your time. It doesn't matter how small/big your business is or how much money you will make. Learning is more important.
• Learn a programming language:
  
  You have not mentioned which engineering branch you are studying. But regardless of your engineering background, it's good to have some programming skills.
  
  
  
  - http://www.codeskulptor.org/
  
  - http://www.codecademy.com
  
  - http://www.code.org/
• Learn LaTeX:
  
  LaTeX produces professionally looking scientific texts with elegant mathematical symbols. It makes makes typing of mathematical symbols easier. It facilitates the creation of structured documents (papers, articles, books, or theses) and therefore lets the writer concentrate on the content of a text rather than on the appearance. Importantly, it is the preferred typesetting system for articles and books by many publishing and professional associations. 
  
  The Not So Short Introduction to LATEX 2ε: http://tobi.oetiker.ch/lshort/ls...
  
  Enjoy your vacation with your friends and family. And remember, career is not your life. Living life is your career.

To read original answer on Quora, click here.