What a great idea to start this thread!
We have two boys.
Our oldest is 13 months and we decorated his room in a sports theme. I did a sports stencil all around the room, made denim curtains with a red sports border, sports rugs, etc. Just too adorable and VERY boyish!
Our younger son is 6 months old and we decorated his room in a transportation theme. I bought a package of stick on pictures of cars, planes, and boats and painted the top of his room in a light blue and the bottom in a light green. I then took chalkboard paint and painted a black stripe all around the room, took a white paint pen and made the black stripe look like a road. Then we put all of the giant stickers up - this is EVERYONE's favorite room!
Good luck to all who are in this fun process!