The general consensus of people who’ve tried this is that being in Japan offers no benefit to learning Japanese and can even be a distraction.  Unless you can already conduct yourself in Japanese, you will find little benefit.  See this link for a few anecdotes.

That all depends on your level of Japanese and what you intend to do while you are in Japan.  But you can anticipate what situations you will find yourself in while you are in Japan and study specificlly the things which will be most useful.

It might be better to make sure that you absolutely know a few things rather than trying to learn too much in a short time.  Knowing a lot but not very well, will not give you the confidence to speak and you will probably end up not even trying out Japanese.  Being able to speak a few canned sentences confidently will make it more likely that you will actually use what you learned.

If you know very little Japanese, get a phrase book and study phrases or questions that you anticipate using.  Basics like “hello” (konnichiwa), “good-by”(sayonara), “please”(onnagai shimasu), “thank you”(arigatou gozaimashita), “pleased to meet you”(hajimemashite), “pardon me”(sumimasen) are good places to start for the absolute beginner.   Learn to recognize the kanji characters for the neighborhoods, restaurants, attractions that you will be visiting and learn to pronounce their names in Japanese.

If you know a fair amount of Japanese already consider reviewing the words and phrases that you anticipate using most and practice listening comprehension.  Also, consider developing pre-made conversations where you can talk at length.  Things like “I am from America and I will be in Japan for 2 weeks.  We have been to some very nice restaurants and I am going to a hot spring Hakone in 2 days.  Have you been to Hakone?  What are your favorite things to do in Hakone?” etceteras.  If you have time, consider practicing conversations with friends or get an account on italki or rhinospike.