I think the newer Bonneville would be a great choice also. A big step up from a 250 but not so much that you are likely to get into trouble. You might end up enjoying it so much that you keep it as a second bike for cruising around even if you choose a much sportier, more powerful bike in a couple years.
There are also some bikes that are no longer produced that you might want to look into if you aren't set on a new bike.
The older Ducati Monsters, specifically the 800 and 900, are awesome bikes, and maintenance isn't nearly as bad as the rumors would have you believe. The biggest mark against them is passenger comfort. I've carried passengers around town or for a 30 minute backroads ride with no complaints, but if you intend on much longer than that it isn't ideal.
There is also the GT1000, which isn't as common, but gorgeous and more sporty than the Bonneville as well
Finally, don't take this the wrong way, but be realistic about how much of a priority what your girlfriend wants is to you. Obviously you want her to enjoy riding with you, but consider if things don't work out and you end up with a bike you purchased because she wanted it, rather than what you wanted.