Overview: Tokyo is massive but incredibly organized—once you understand trains + neighborhoods, the city becomes easy (and addictive) to explore.
Plan your days around clusters (e.g., Shibuya/Harajuku, Shinjuku, Asakusa/Ueno, Ginza/Tokyo Station, Nakameguro/Daikanyama, Shimokitazawa, etc.), and expect to do a lot of walking even when you're taking the subway.
**Visas:** US citizens don't need a visa — you can stay visa-free for up to 90 days. Your passport just needs to be valid for the entire duration of your stay. Japan doesn't require the 6-month validity rule that many other countries do.
Transportation: World-class transit. Use IC card (Suica/PASMO) for everything. Two subway systems (Tokyo Metro + Toei) plus JR lines. Trains beat taxis for time + cost. Last trains earlier than expected—plan accordingly. From Haneda: 30-45min. From Narita: 1-1.5 hours. Consider luggage forwarding (takkyubin) between hotels.