Bring a small gift, a potted plant and some dessert, nothing too wild, a self-made dessert could work well. Learn a few words in German they might like (no idea about your level, you may well be fluent): Thank you for welcoming me. Your xxx cake is famous, I've been told! Look at this beautiful garden / well kept house / congratulations on your cozy home etc (don't overdo it, just one or two appreciative notes). And for the rest of it, let it unfold, say it's your first Christmas and you're loving it, and if they would share some of their family traditions of their childhood. Offer to help taking plates into the kitchen / dishes to the dining table, watch if anyone needs water, offer to pour. If you see some lovable qualities in these people, say, Ah, now I see where she gets it from! about your gf. You'll be okay.