I've been across much of Japan back in the 1980s. Hiroshima for the museum, peace bells and such. Over the hill from Hiroshima is Iwakuni, both a joint Naval Air base and also the site of Iwakuni castle. Good little Feudal Japan museum there. Matsushima in the north is interesting and has a good temple. Tokyo is good for a day or two just for the sights and sounds. Mt. Fuji is a popular suicide point for Japanese. Consider going there.
Don't expect the food to be like Japanese restaurants. Their staple food is noodles - Soba. Tofu too. Meat is extra. You'll find out why Japanese are a lot skinnier than Americans.
Their laws are strict on misconduct, drunkenness, etc. Don't violate any laws, mind your manners. Don't mistake politeness for weakness because they aren't weak.
Learn basic phrases in Japanese, what I call "restaurant and bar" Japanese. Japanese learn English in school so they speak at least a little. However, if you're the "ugly American" showing no respect for a shop owner they'll decline proving they know English. They're an honorable people who, on average, would rather lose a sale than look stupid in front of an Ugly American.
OTOH, if you are friendly, polite and attempt to speak Japanese, meaning you humble yourself first by looking ignorant, the smiles come out and they'll be equally polite and helpful. Between the two of you, all your questions should be answered.