When we first moved into this house we had no living room tree (later bought a slimline, pre-lit one in the sale).
I bought a £5 garland from B&M (was essentially twine with pine cones, berries, wooden stars and some Holly on it), attached some extra greenery and wrapped a set of berry lights around it. That on the mantle piece with a little red lantern with battery tea light candles on either end looks very festive.
Lights in the window, candles in festive jars/holders and touches of greenery are always nice (tucked on the corner of pictures on the wall etc), plus cushions and blankets (not necessarily Christmassy but tartan/green/red etc).
Or you could always go the more is more route, hang the old school foil decs from the ceiling, paper chains, bunting, fairy lights hung everywhere etc.
Not everyone's cup of tea (and wouldn't suit every house) but our old house was Victorian and had very high ceilings.
One year, to cheer me up, DH hung a set of fairy lights from our hallway ceiling, having them crisscrossed from side to side.
Sounds weird but the effect was really nice 