Some of the cutest rooms I've seen had no themes, but I had such a good time with ours. We're re-using our daughter's nursery, which I painted Small World, for our son when he is born. When she was a baby our daughter used to wake up in her crib and babble for the longest time to the murals on the walls. We used to spy on her. :lol: She still likes to go in there and pretend she is on the ride, taking her stuffed animals through the different countries. She likes to find the depicted countries on her map, run in circles singing the song, teach her dolls to sit still on the boat, just has all kinds of fun with it.
Her new room is tropical themed, and I did murals of Lilo & Stitch, Squirt from Nemo, etc, and one wall is painted like a giant window looking out onto the beach. At night we take turns telling Magic Window stories, in which she crawls through the "window" and has all kinds of adventures. I have to say she is getting mighty creative in her storytelling for a person who is not yet 3. :lol:
You don't need a theme to have a good child's room, but if it's something you can do yourself that won't cost a lot of money, it can be a lot of fun.