Welcome, Sylvia! I am your host, your 'ghost host'.
Only joking, it's just me, Sarah! Sorry if I frightened you prematurely.
This is such a fun question. Last August I took my son to Walt Disney World for his 5th birthday. When he woke up on his actual birthday he was surprised to find a decorated window. I purchased a Mickey-themed birthday decoration set before our trip and brought scissors, balloons, and tape in my suitcase to surprise him. I read somewhere that tape wasn't allowed on the walls due to the possibility of chipping paint, but I attached a few balloons to the walls and no one said anything to me about it. I also hung some string from the top of the window with Mickey shaped cutouts attached to the end as a sort of garland hanging. Mousekeeping left our decorations alone and even added little towel creations to the party in our window.
I have seen pictures online of people decorating their Resort Hotel window with cool holiday themed lights and window clings. We stayed at Disney's All-Star Music Resort and saw just a few other decorated windows during our stay.
I would say that as long as you aren't puncturing the wall structure or causing damage to the wall or window, you should be free to decorate. Also, you probably shouldn't use any adhesive that is too sticky. Clear scotch tape worked perfectly for me. It was easy to remove, and left behind no residue. This is not official information, just an opinion based off of personal experience. You might want to contact the front desk if you have any specific questions.
I hope you have so much fun decorating your spook-tackular window. Be sure to stop by
Memento Mori at the Magic Kingdom to pick up some frightfully awesome trinkets to 'liven' up your window. Hurry Back!