Asakusa's Sensoji Temple is packed with well-wishers on any day, but more so for hatsumode on New Year's Day. Hatsumode is the act of visiting a shrine or temple during the first days of the new year to receive good luck and fortune. Sensoji is one of the most popular temples for this tradition – in fact, the nearby streets are blocked off to accommodate for the swelling crowds.
You'll be waiting in line for a while but don't worry, you'll be surrounded by shopping stalls selling the latest new year memorabilia and souvenirs, and when you reach the temple, throw in a ¥5 coin and make your wish.
For a marginally milder crowd, get there on the evening of December 31 to participate in joya no kane, a Buddhist tradition where a large bell is rung 108 times at midnight, each ring to get rid of a worldly desire, kind of like a sin.
Do note that while the temple is open 24 hours, the shopping stalls have varied closing times, usually around 6pm.