How to get the most out of an internship (in industry)?
Let me add two important items to eykanal's great answer:


Do awesome research.  If you want an academic career, you can't ever not do this.  A good internship is not a vacation from research; it's an opportunity to broaden your research portfolio.
Cultivate references for your future job search.  Find, work with, and impress people whose interests overlap yours and whose opinions are valued in the academic research community.  These people may work at the company, or they may be visitors.


When it comes time to look for an academic job, your application will be significantly stronger if you have recommendation letters from and publications with people who don't work at your home institution.  In fact, if your CV lists an internship, but you don't have a letter from or paper with someone at your hosting company, that gap will raise a (small) red flag with recruiting committees.