well for electronics and latest gadget.. there is only one place "AKIHABARA"
for girls, latest fashion , "SHIBUYA, SHINJUKU, IKEBUKURO, HARAJUKU, GINZA"
Ueno is also a great place to visit, lots of food , very cheap things you can buy and its just beside akihabara.
And don't forget to try Ichiran Ramen, there is a shop near Ueno Station.
Also in between Aki and Ueno, there's an eat-all-you-can Yakiniku restaurant called "Carne Station" but there is a time limit only 1 hour.
And if you love spa or onsen, and mountain
you can try to visit "Hakone" . There is a round trip ticket , i think you can buy it in Shimbashi or was Shinjuku
( sorry I forgot ).
Night Life , Roppogi ( Hills ) is your best bet.
Yokohama is a must place to visit to
Anyways .. Enjoy