Early one November morning - about 5 AM - I started a walk from Nagano Station toward its most famous temple.
Zenko-ji was founded in 642 as an independent temple, when "The Image of Sangoku Denrai" came to Japan from India, via Korea. Apparently this image has been hidden since 654, and nobody has seen it in the millenium and a half that has passed.
At 6:32, people could enter the temple and sit on the tatami mats, and observe the morning service. You start in the Hondo (Main Hall) and then you visit the Naijin (inner sanctuary), which is where you sit to observe the morning service. Your final stop inside the temple is a tour, or walk, through the O-kaidan, a pitch black passage beneath the temple. You find your way through by feeling the walls. Near the end of the passage you hopefully can feel a wheel which you spin for good luck.
The ceremony itself, inside the Naijin, involves a lot of synchronized chanting; the abbot does some blessings of images during this ceremony, which is a little difficult to see, but most of it will just be listening to the chanting. This was my first Buddhist blessing and first service, though it was my second pilgrimage.
If you visit for the morning ceremonies, note that you will sit on your knees or cross-legged ("Indian style") for a long time, and your tour through the O-kaidan will be slow and crowded because those who watch the morning ceremonies will all take the pilgrimage. If you're not interested in the morning ceremonies you should visit later in the day. There are three more O-juzu Chodai blessings during the day, though their actual times can vary.
It is possible to do a temple stay here, living life as a monk for a while and eating monks' food. I don't know the details of how that can be done or any costs (monetary or labor-based) that includes.
Zenko-ji is open 24 hours a day, though as mentioned before, the main hall opens around sunrise until about 16:00. The Kyozo (Sutra Repository - apparently this building closed right now for maintenance) and museum are open 9:00-16:00. Admission to the Naijin-ken, O-kaidan, and history museum is via a 500-yen combination ticket.
To get there from Nagano Station, I recommend walking two kilometers north along Chuo Dori (a slightly hilly path), which takes about 30 minutes. You'll pass by the above monument and be able to see the Olympic site, and get a good bit of exercise. You can also take a bus from Nagano Station; use the Zenko-ji Exit and take any bus from Bus Stop #1. It takes about 15 minutes and costs 100 yen; from the Zenkoji-Daimon bus stop it's a 5-minute walk to the main hall through the gates as you see above. If you want to go to the temple very early in the morning, like I did, you will probably have to walk from the train station.