The activities depend on age group and location, but here are the most common activities:

1. Countdown: for the youth, there are countdowns at places like Tokyo Disneyland, Universal Studio Japan, etc. I have been to the countdown at Tokyo Disneyland. Expect huge crowds!

2. Visiting temples to ring temple bells for the new year: people of all ages line up at temples to ring temple bells. Quite fun if you are a tourist, and you have company to talk with while queuing.

3. Hatsu Hinode (New Year's sunrise): people gather at a range of places, like Tokyo Tower, Mountain Peaks and parks near the sea, top see the first sunrise of the year. Great photo opportunity.

4. Visiting shrines to get blessings: During the first three-four days of the new year, people visit shrines to get blessed for the new year. You can also write Ema (wooden placards with wishes)b at shrines. Expect long queues.

Most train lines operate overnight on the new year's eve, so that people can attend these events.

If you live in rural Japan, people have regional events that are more interesting. Do check with people around you.

Those who do not feel like going out in the new year's eve, there is "Kouhaku", the NHK year end concert. I did not find this very interesting, but it is a good way to get a summary of popular songs of the year.