Flowers are great - but at this time of year, bring a "ready made" arrangement already in water - not something that needs time to put in water or arrange on arrival. (I know it only takes a couple of minutes, but if you are a frazzled hostess, that can be a couple of minutes you don't have).
Scented candles. Or ones with lovely designs on them.
Plant in a pot - poinsettia are lovely, or mini roses, or ivy arrangements and all are easily found even in supermarkets if you don't have a handy garden centre nearby.
Christmas ornament would be nice - Newbridge silver do lots of different ones between €12-€20 in value, and you could find one that suits the personality/family/hobbies etc of the hostess. Or a good local craftsperson may have nice ones too.
A Christmas bonbon dish or similar? If you can't bring chocolates, the hostess could use it for her own ones.
Seasonal pot-pourri (if you know hostess likes that kind of thing), or reed diffuser.
If you can't bring wine but know they like it, perhaps something like a nice wine pourer or stopper or other accessory? Or similar for cheeseboard, a cheese knife or set of them?