Zenkoji is certainly an important landmark. It was founded in the seventh century, before Buddhism divided into many sects. Currently, Jodo Shu (Pure Land) and Tendai priests run things there. I stepped into the main hall around noon, when a service was being conducted by Jodo priests, including recitation of the Nenbutsu chant to Amida. A lay volunteer approached me, speaking very good English, to explain some of the features of the temple. A bell rang, and he said 'You are very lucky, the curtain on the altar is opening.' I could see the curtain opening, but not much else. The curtain closed immediately. It is very dark, and the main image of the temple is a 'hibutsu' or secret Buddha, meaning that it is not displayed to the public. In this case, even the high priests have not seen it. It is in a backpack-type bag (for quick evacuation in case of a fire or something) inside two more box-like altars. So I may have seen the outer box or something. The statue is said to have come originally from India, by way of Korea. There was a feud or struggle of some kind, and in the kerfuffle, the statue was thrown into a canal. A man named Yoshimitsu was directed by some kind of vision to find the statue in the water. He retrieved the statue and established the temple, which uses the kanji for his name, pronounced in the Sino-Japanese way.
Another famous feature is the dark underground passage. My informant asked me to confirm and keep in mind the location of the altar. Then, he pointed out a stairway leading down into darkness. Keeping your right hand on the wall, you descend into the darkness, which represents death. Keeping on into the darkness, and remembering the location of the Buddha, you can find a door right beneath the altar. Because the Buddha is in the Pure Land, this door could be considered the door to paradise. There is a heavy old-fashioned lock that you can rattle and manipulate to express your desire to get there. Then, because you are not really dead, you ascend back into the world of light. There is a mirror conveniently placed so you can see if you changed in the process of symbolic rebirth.
My informant also recommended a visit to the three-story pagoda in the garden. The building is concrete, a bit of a disappointment, but it has many interesting things inside. Possibly most important, the Dalai Lama came to Zenkoji and made several offerings including a sand mandala. This mandala was made to be permanent--the video shows the monks using what looks like glue on the surface where the sand will be sprinkled. Zenkoji pulled out of the 2008 Summer Olympic torch relay in protest of China's Tibet policies. This may be one reason why the Dalai Lama came to visit. In addition, you can get close to some very old statues, and view explanations of some of the preservation and restoration processes. There are several old plaques which I didn't understand well--maybe offerings of some kind?-- and also a room where you can do 'shakyo' or sutra copying.
This temple is definitely worth a trip, if you are a hard-core Buddhist. It is fairly easy to get there from Tokyo, either on the Nagano shinkansen bullet train, built for the Nagano Olympics, or a highway bus.