Haven't been to Guadalavaca so can't really comment on that but ....
Havana would be a fantastic spot for photography. I really regret that both of the times I have been there was well before I got seriously interested in taking pictures. The only thing is, you are going to have to fly there because you are based at the other end of the island and it's quite a distance.
I spent a few days in Baracoa (which is near where you will be) and ended up having to get a flight on one of those tiny ex-Soviet prop planes to get out of there and back to Santiago. Quite an experience but not one I'd be in a hurry to repeat ....
Speaking of which, Santiago De Cuba is really good as well. It doesn't have the grand elegance/architecture and sheer variety of things happening on the streets that Havana would have, but it's got great life to it, and if you are into the music it's the place to go ...
I would second the previous posters comments about Cuba being all about the people too ...... but also the scenery .. and the buildings .. and the history ... and the politics .... and the music. More or less everything except the food in fact.
In short, I think Cuba is photographic heaven. Try not to drive your girlfriend completely nuts by spending
all of your time with a camera shoved in front of your face