Have a look in somewhere like Poundland and see what things they have to hang on walls/windows - you could pin things with safety pins to curtains if you can't stick things to the walls.
Bring some battery operated lights or even 1 string of electric ones, and put those on the fireplace or around 1 doorframe, or as 1 focal point.
Print off some festive colouring sheets and activity sheets from places like Activity Village, DLTK or Santa Update - for the DCs to do in the house and you could put up there.
If you know if there is a bath, maybe a festive bath bomb for the DCs (Lush do nice ones), or if you don't know, perhaps bring a nice shower gel instead. And do bring a few things to be able to relax yourself there - whether festive or more indulgent.
If you generally bake at Christmas-time, weigh out the ingredients for something you like to do, bag them up into a "kit" and bring the tin as well, to bake together in the cottage (or that you can bake when the DCs are gone to bed to relax for yourself). Or maybe, make (or buy) the plain cookies and bring those along with readymade coloured icings and various decorations etc - to have a cookie decorating afternoon.
And have a look at the websites for the area to find out if there are any special events going on there, a Christmas market, local church services (if you'd like that), a Christmas Day Swim (to watch!), etc, while you are there. And also to find a nice place for going for a family walk on dry days.