[samurai007]

Budget: If you're frugal, that should last you considering it doesn't include hotels or JR trains (because of the pass). But it'll be a bit tight, especially if you go out drinking, as alcohol can be expensive in Japan.

Weather: It'll be spring, but the years I was there, spring tended to be fairly short. So it may still be chilly, it may be quite warm, or it may be nice spring temperature. Not sure. I'd check weather forecasts for Japan before you leave and pack accordingly, taking at least 1 light-med jacket for cooler days and evenings, and some t-shirts for warmer days.

Trains: There are some JR trains within some cities, but subways tend to be very useful too (and they aren't covered by the JR pass). A subway ride is usually only about 200-300 yen though.

Tokyo nightlife: Can't help here, I never went to any night clubs in Tokyo, and besides, I was in my mid 20's when I was in Japan, so I'd have gotten in no problem anyway.

Recommendations: Look over some of the Travel Report threads for places to go and things to see. There are some common recommendations, like Nikko, Hakone, Nara, Himeji castle, Miyajima Island, the various temples in Kyoto, Koya-san, etc.