I've been to Japan a couple of times. Once with my Father and the other time with friends. I enjoyed it so much more with my Father - I think it's such a fabulous place for a family holiday!
Kyoto has heapssssss of temples (like 2000ish I think), so you're bound to be cheking out a temple. Check out the Golden Pavillion, Silver Pavillion and the former Imperial Palace.
You have to go to Ueno Zoo to check out the pandas, so cute - and of course Disneyland in Tokyo. If you want to head out, 'Ropppngi' is the area with all the bars/clubs etc (I think thats what it is called..). You can bathe in the Onsens (hotsprings) if you like, but everyone goes butt naked, so it can be a little bit daunting! But yeh, if you want to do that check out Ooedo Onsen Monogatari in Odaiba its basically a hotspring theme park kind of setup. You MUST MUST MUST try and find mr donuts, seriously awesome donuts. Makes me want them just typing about them. You may want to get hold of a katakana chart, will make ordering food over there so much easier, otherwise jus tpoint to what you want!
Theres nothing much in Hiroshima apart from the Memorial Park, and thats a must..
Anddddddd, theres things called 'love hotels' where you can go to, ummmm, have sex
Enjoy it and I hope whoever you are going with doesnt drive you nuts, cause Japan in itself drives me nuts!!!!
If I can remember anything else ill update my jumbled post for you.