Hi Danielle,
if you guys aren’t into castles and won’t be in Osaka, I definitely would skip Osaka Castle. It looks amazing from the outside, but is a tourist shop on the inside. While there aren’t any noteworthy castles in Kobe, Himeji Castle is only an hour away by local trains or around 20 minutes by bullet train. Himeji Castle is one of my favorites. It looks stunning from the outside, and is amazing on the inside. Just a fair warning that because the inside is very “authentic” the stairs used to go up and down the different levels of the castle are very steep and/or small. So if you have trouble walking or don’t like small spaces, the inside of the castle might not be for you. Also, if possible, try to go on a non-national holiday weekday. National holidays and weekends are very busy, so if you can’t go on a weekday, make sure to get there early.

A couple of my other favorite castles are Matsumoto Castle in Nagano and Kumamoto Castle in Kyuushuu. Unfortunately, Kumamoto Castle is under repair since it was heavily damaged by an earthquake in 2016. Hiroshima does have castles too, but Miyajima Island and the Peace Park/Museum are much better places to spend time at, in my opinion.