Usually, what you'll be able to do is to apply the summer courses. For example, I applied for Harvard Summer School. Check out their website, and you can find all sorts of interesting courses they offer for a summer term, which usually lasts about 6~8 weeks. The application process is similar to the ones you had when you're applying for grad school here (TOEFL score, health condition and so on).BTW, the on-campus dorms' rent is as high as $4000/ six weeks. If you're thinking to take a summer course, I would suggest you come early and search the Craigslist for a short term sublet, and it usually cost between $600~$700/month.
I'm taking International Marketing and Cross-border Innovation this summer. I would say there is a lot of effort and work required as a regular four month semester class. The capacity for Havard summer school at Graduate level is huge, for example, for the marketing class, we have 80 students from various parts of the world, Germany, Egypt, Mexico, Denmark, Switerland and more. You'll meet with graduate students at the other side of the world, or business people who are seeking some new ideas from the prestigious university. It's been a week since the first class, and so far, I've been enjoying it.
Another great advantage besides having the classes is that you can access to most of the libraries in Harvard. The famous Widener Library is one of them. The Library is named after Mr. Widener (right), and the interior is more like a museum, as I may say. And I'm pretty about the place where I'll spend most of my July time. See it for yourself! I wish I could take more pictures, but the reading area is photo-forbidden.
If you're coming here for your graduate program, I really suggest you take a summer course at a different university, and the credits are usually transferrable. Harvard is a good choice, and Boston University also offer some great programs, like the New Media Course by Professor Steve Quigley from College of Communication : )