Oh I get it; a few years ago we were supposed to spend 6 days in Kyoto after a week in Tokyo. After 3 or 4 days (including 1 in Osaka and Nara), we hopped on the train back to Tokyo to finish the trip.
Adding on Hiroshima would be pretty easy as you suggest. Adding on Miyajima would require a bit more planning.
Edit to add: I’m sure you have all your big reservations (if any) planned for Tokyo, so below are some walk in noodle/other places I can recommend (as of like 5 years ago, so COVID closures may apply).
Tatsunoya Ramen - amazing smokey garlic tsukemen. My first meal off the flight extremely jet lagged. Order with vending machine.
Shin Udon in Shinjuku
Afuri ramen - yuzu based ramen. They have a few locations, all in trendy locations.
Fu-unji ramen - long lines but fast and terrific
Nagi Ramen - in Golden Gai. Anchovy (or sardine?) based broth
Butagumi - marbled pork katsu.
Nodaiwa in Ginza for unagi
Tempura Tenfusa if you want eel tempura.
Jazz Blues Soul Bar was a dope record bar we went too. Highly recommend doing a record bar if you can. This one (maybe all) will have plenty of tourists.