The island Miyajima, near Hiroshima is romantic. Lot's of nature, temples, and some nice hotels... however it is expensive!
I think that Kobe is also pretty romantic, especially if you have your hotel near the Akashi kaiyko bridge, in the evening you can see the bridge lightened up in 8 different colors witch you can view while you are having a nice romantic walk on the Boulevard (don't forget your small fireworks!)
Another nice place also in Kobe is Mt. Rokko! have a nice walk in the afternoon in the mountains, or spend some time at the small attractions on the top, have some great dinner at the Rokko garden terrace ( View over Kobe and Osaka... an amazing view!) and then go to the lovely hotel where you can take a hot bath with the same view from the Garden terrace!
This are some of my tips for the moment, hope you like it!