As a fellow "non-professional" photographer, I LOOK for opportunities to travel and take interesting pictures. Every trip is different, so I always carry a "tailored subset" of my equipment -- usually in my Lowepro CompuTrekker backpack. (I even take my gear on business trips -- just in case.)
I lived in Japan for 9 years, and I've traveled extensively in the Wash, DC area. Of course, no destination is crime-free, but I consider Japan to be one of the safest countries in the world - I never felt threatened, or worried, for my gear in Japan. As others have mentioned, Wash, DC maintains a high level of security everywhere. In short, insurance, good situational awareness, and common sense will keep you and your gear safe.
In both locations, I "anticipate" that you will be doing a lot of walking, and riding a lot of subways & trains. So carry-weight may be a concern. I think your wide angle lenses will serve you well when visiting Japanese temples, shrines, flower gardens, etc. -- also useful for the monuments, memorials, and museums in Wash, DC. I'd also advise carrying a decent rain sleeve of some sort.
{FYI, the cherry blossoms (in Japan) "generally" start blooming in mid-to-late March and they last about two weeks, into early April -- but a LOT depends on the weather. A warm, early, Spring can cause them to bloom early. Extended cold weather can cause them to bloom late. A heavy rain storm, or strong winds, in the middle of the blooming season can really beat up the delicate flowers. So your chance of seeing cherry blossoms depends on the weather, your dates of travel, and your location in Japan. The blooms travel in a "pink wave" starting in Southern Japan and moving North.}
Good luck with your trip! 