A city you can visit over and over again
Nothing much new can be said about Tokyo, and by large you'd have to really explore it yourself. It will take more than one trip! Being one of the great megacities of the world, if you are strapped for time, visit the places that make it so - Akihabara, Shinjuku, Harajuku, Shibuya, Ginza, etc. Tsukiji market is always interesting - but if other parts of Japan make up your itineary, in all honesty the temples, parks, and 'traditional' japanese places in Tokyo will pale in comparison.
Insider Tip:Once you get in, get a PASMO card from one of the vending machines at any subway/train station - makes it easier for you to not have to figure out machines at different stations. For simple data uses (maps, whatsapp, google searches), if you are travelling solo get a 14-day SIM Card at around 3000yen - in a big group, rent a pocket wifi. Data is your friend when searching for train directions.