My DS was 5 when we went, and he put a great deal of thought into his costume. (He's knight-crazy, so he had the dress-up clothes already.) He wore a Narnia shirt, a metallic-material cape and helmet made from the same material (so it would pack flat) with red plumes, and a sword. He was the only little boy dressed up (his cousins were going as Belle and Cindy, so he didn't feel awkward), and it paid off big. The princesses thought he was adorable, and we have a great shot of Cindy giving him a smooch on the cheek for his picture. I didn't even mind "having" to buy that one.
Another nice thing: We had everything we needed already, and it cost much less originally than my nieces' princess get-ups. (They looked absolutely precious, of course.)