If they’ve traveled extensively together, I really don’t think they’ll need a travel agent. I work for in the travel industry and used to live in Japan, here’s what I’d do if they want to do it without one.
Do they have their flights yet? Use google flights and Look at both Tokyo and Osaka airports. Remember it takes roughly 2 days to get there and two days back so you need at least 10 days of travel.
Use the Japanese national tourism board site. Great resource.
Use TripAdvisor forums. They are invaluable and full or recent trip plans and helpful details.
https://www.tripadvisor.com/ShowForum-g294232-i525-o135180-Japan.html