Things we've done:
We've brought a card stock (flat) tree that we hung on the wall with a push pin. It came from a school supply. We also strung lights around the window. We used little peel and stick hooks to hold them up. They came right off the glass when we left.
One year at BWV. We went to Wal-mart and bought a small (~30") fiber optic tree and set it on the small round table. There isn't a lot of extra space in the 1br. We just put it back in the box and checked it on the way home.
Another year at OKW, we bought about a 5' tree locally. There is plenty of extra floor space at OKW. We brought lights and garland from home. In the first couple of days at WDW, we bought plastic character key chains and just hung them on the tree. They make nice souvenirs too.
(Wal-mart and Christmas tree shopping are just two more reasons to have a car at WDW.)