There seems to be some disagreement here, but myself I would pick either Kyoto or Osaka and get a hotel for 4 nights in one or the other instead of splitting it. The two cities are close enough that you can take an early morning Shinkansen between them and return on a late night one to your hotel.
Although it sounds like OP is just looking to relax in Osaka for a night so I suppose the move between hotels works out better for them.
One thing I have to add about Kyoto, is there's a K'loop sightseeing bus that stops at the major attractions but I found that it either gives you too little time at some or too much time at others between buses and did not feel like it was worth taking it unless you find a place on the line that you can't get to otherwise. I was able to get around much better with the city buses, trains, subway and a little walking.
May be a little overdone, but I would hit up the major places like Arashiyama, Nijo castle, the major temples and Fushimi Inari shrine. The sake distillery tours are nice as well.
Iwakuni was nice and easy to get around with attractions in Kikko park in the Kintaikyo and Iwakuni castle area,
I went to the Hirasei for lunch which served iwakuni sushi and set meals with various renkon dishes.
Also early happy birthday.