I found this super cute idea- so petsmart has these red + green willow balls in a four pack, you can string them on a safe type of string and then hang them!
You can also take some coloured napkins and shred them to use as a bedding.
I know Rosewood naturals has a lot of Christmas themed items so perhaps check them out- I know they're a UK based brand but you might be able to find them on amazon.
Toilet paper/paper towel tubes are great- you can cut each end and then fill it with hay, treats, etc and then twist each end off so then it looks like a Christmas cracker!
For decorating the outside of the cage, garlands, lights and stickers are always nice 