princess, depends on which part of japan u gg. tokyo or hokkaido.. when r u gg? if early april, its better to go tokyo where u can see cherry blossoms.. if u gg later..shld go hokkaido..
anyway, u will need this website for train rides in tokyo if u r gg F&E.. for planning time & cost.
www.hyperdia.com
u can go asakusa, ueno park, tokyo dome, yasukuni shrine, imperial palace, tokyo tower, shibuya, shinjuku, disney in tokyo...
take a train ride out to osaka for osaka castle & universal studio..
and u can go kyoto too... very beautiful place.. highly recommended to see cherry blossoms...
there's philiospher's walk, kiyamizu temple & various others...
u shld buy the JR pass too .. tt, u can only buy it outside japan.. tt means u buy in sg...
hope tt helps. =)