Depends on what you like!
But when I went there I thought:
1.
Sumo match! - Really cool seeing guys 5 times my width throwing each other around!
2. Harajuku station/park - People watching! Some really interesting characters - semi-goths, punks, all sorts!
3. Meiji-Jingumae Shrine (just down the road from Harajuku station).
4. Asakusa shrine - Huge tatami slippers, and a huge drum!
5. Odaiba - With the futuristic Fuji building, and a mini statue of Liberty!
6. Tokyo-Edo museum (if you're into that) - cool recreations of Tokyo's history.
7. Philippe Starck's Asahi building - near Asakusa, supposed to look like a glass of beer.
8. Sony building in Ginza - if you like technology.
9. Akihabara - also if you like technology.
10. Ueno park with the museums/Imperial palace/Shibuya!