We like to do theme birthday parties at our house. We have had a variety of them over the years. With girls, it seems a lot easier to include costumes as part of the theme. Last year, it was princesses and everyone got princess dresses to wear. For my son, it can be harder. Trying to find costumes for boys to fit the theme is not always easy. Last year was Phineas and Ferb, I'm not sure what I could have done for costumes there.
This year, it could be different. Harry Potter is the theme that my son chose for his birthday party (any ideas on a cake? - this is the first theme chosen by my kids that I am not familiar with). There seem to be a number of different Harry Potter costumes available. My son of course would want the Harry Potter one, but you can also find Gryffindor robes, Quidditch robes and even Voldemort (now if I had read a book or watched the show, maybe I would know the difference and could come up with a theory on what type of robes we could use. There are even accessories available too.
Even if we don't dress up at the party, my son has decided that the Harry Potter Robe would be part of his Halloween costume this year too. He will wear his shirt and tie from First Communion. We still need to pick up a wand and some glasses to finish accessorizing, but we have a few weeks until his party and Halloween. Here he is in just the robe: