There are lots of things to see in Japan.
Tokyo, Osaka, Kyoto would be the major attractions so if you are a first time traveler to Japan, those cities are places you might want to check. Tokyo offers multiple things to do so it is recommended to plan according to your interests. Kyoto is ex-capital of Japan with much longer history comparing to Tokyo. Exploring old streets and beautiful temples and Japanese gardens gives you feelings of the beauty of Japanese traditional culture.
A lot more attractions are in rural areas, beautiful nature, old houses and buildings, great local food, etc. My recommendation is Tohoku area. They have great nature, temples, food, and Japanese sake. Every part of Japan produce Japanese sake, but especially those from Tohoku and Niigata Prefecture are famous.
Every place is enjoyable even if you are a solo traveler. The only problem is you can not get yourself in pictures. That will be solved with selfee sticks or asking people for pictures. Japanese people are generally kind so they will happily take a picture for you. You do not have to worry about your camera be stollen, too.
Interesting thing in Japan recently, especially in Tokyo, market for solo or singles "Ohitorisama" is getting bigger. There are places where people can enjoy Yakiniku (Japanese barbecue) alone (each person gets Shichirin charcoal stove!), Karaoke for solo (the room is very small!), ramen restaurant with walls between people (!), etc.