I hate to break it to you, but cramming generally doesn't work very well. That said, if you don't have a choice (and I'll assume you don't), here are the easiest places to cram:
1. Learn the few most common sentence correction error types.
2. Review your basic math, such as number properties, algebra and geometry rules, etc.
3. Get a prep book and practice some of the basic tricks and shortcuts, such as plugging in numbers for math.
4. Do problems from the Official Guide, focusing on the middle 50% of each section.
5. Make sure to take at least a couple practice tests (the GMAT Prep tests from mba.com are best) to practice your pacing.
Don't bother studying the hardest stuff, since you don't have time and you're unlikely to see it unless you're scoring 700+. For example, feel free to ignore permutations and combinations, except maybe the most basic formula. Reading comp is also a tough place to cram.
Good luck!
Jim S. | GMAT Instructor | Veritas Prep