Next to her archway where she enters would be this cute little mailbox from Pottery Barn Kids. So her mumma could leave her special little pieces of mail throughout the day!
I would place this adorable Hello. decal from TastySuite right by the mailbox.
For lighting, I would hang a large paper globe light. (I must admit that I somewhat stole this idea from my neighbors...they have a few of these hanging in their kid's playroom, and I just love them!)
Those are just my thoughts so far. I'm still looking for some fun artwork for the walls. It would probably be in my husband's best interest to get this project going sooner rather than later as the longer it goes, the bigger my ideas are going to get!