We basically didn't see anything famous on our trip. Most of the sites in Japan get rather boring. They sell the same souvenirs and have the same stuff. We did ride to the top of one nice hill (mountain?) just north of Akita. Kanpuusan = Mt. Cold Wind. It gave us a nice view of the sea and surrounding area.
We would stop now and then along the beach to see a shrine or jump in the water. In Hokkaido there were many dairy farms selling ice cream and whatnot. Otherwise we generally stayed away from anything famous because the traffic is bad and things are too expensive.
We did make it as far as Sapporo where we took a rest on Oudouri (Big Road). It has to be the biggest and most famous boulevard in Japan. They have a huge ice carving festival in the winter, but in the summer it is (what else?), beer gardens. We also ran into a famous chocolate factory by accident. We didn't take the tour, but I did buy a great vanilla chocolate candy bar.
We averaged about 125 kilometers per day. This is not a particularly great feat for one day. We tried to get up early each day, but it was not easy. Then it would get hot in the afternoon and we would take a nap. We rode till about six each night. Considering we average from 20-25kmh when we were actually on the bike, you can imagine we spent a lot of time off the bikes.
I had a decent road map of Japan that we used to navigate. We took mostly national highways, two lane. We were able to navigate amazingly well. We soon figured out that the coast area was flat and cool while inland would suddenly become a hot mountain pass. We spent lots of time jumping across from one bay to the Japan Sea to the Pacific Ocean. This kept us out of traffic and gave us some varied scenery.
Most drivers were pretty considerate, but we did get a few annoying honks. There was not always a decent shoulder, so we often road with the cars. If you don't like big trucks, then Japan's highways are not for you. Otherwise, I think it is generally safe and enjoyable. Try to avoid a lot of tunnels unless you are fast and have a good rear light.