[NTREEG]

You may pick up a few phrases but I doubt your Japanese will substantially improve only from being in Japan unless you get a lot of practice from other natives. In my opinion, the best way to learn a lot of Japanese fast is to enroll in a Japanese Language School in Japan. I've been attending one for the last 7 months now and have gone from zero ability to being able to participate in simple conversations. Many schools' classes are broken up into 3 month quarters so it should fit your schedule pretty well. If you're anything like me, you won't retain anything very well until you actually use it in real life situations and conversations outside of class. So you'd do well to find some language exchange partners to supplement your classroom studies.

To maximize your rate of learning, I'd recommend learning to read and write hiragana and katakana before coming to Japan and starting lessons. Also try to cram as much vocabulary as you can before coming for the summer. The classes are great at teaching you grammar and drilling grammar points. Vocabulary you really need to study (memorize) on your own. Once you start learning the grammar, you'll really want to have a base of vocabulary to call on so you can use this new grammar in sentences.

As for books, I used Genki I at home in the USA before coming to Japan. It was useful, but the books we use in language class is an entirely different beast. They're written completely in Japanese with no romaji and no English explanations (which is why you should learn hiragana and katakana ahead of time). It's really intense but you get used to it quickly and you'll surprise yourself with how fast you progress. Oh yeah, also pick up a really good electronic dictionary when you get to Japan. I first bought a "world traveler" type elec. dict. in the States but it was horrible. Finally picked up a top of the line elec. dict. went I got to Japan and it's one of the best investments I've made.

Here's the books I used for the first 6 months:
Shokyu Gogaku Ryugakusei no Tame no Nihongo 1 (2001 Bonjinsha) Shokyu Gogaku Ryugakusei no Tame no Nihongo 2 (2001 Bonjinsha)
語学留学生のための日本語

Good luck to you.