You would need:
- Fun outfits - old costumes, uniforms, suits and dresses, perhaps with a chosen theme.
- A selection of fun accessories, like hats, wands, tiaras, or boas.
- An old trunk, quirky storage box, or suitcase to hold your finds
For themes, think:
Western - look for bandanas, cowboy-type shirts, hats, jackets, and boots.
Sci Fi - you want alien-like accessories and gear (My son wears a man's shirt of the right color to pass for a trenchcoat like the Tenth Doctor's from Doctor Who).
Fairy Tale - a selection of witchy outfits, prince and princessy outfits, fairy wings, pointy hats, and related accessories. (My daughter has two cauldrons that were once Halloween decorations).
Super Hero - Capes, masks, matching long-sleeved shirts and sweats or leggings, or any of the many, many hero themed costumes released for Halloween.
Animals are a favorite theme around here, and I've been lucky to find lots of high quality animal costumes while thrift shopping.
Glamour - sparkly, feathery, flowing, silk and satin garments and funky sunglasses fit for a movie star.
Professions - like doctor's labcoat, nurse scrubs, ballerina tutu, police and firemen's uniforms, that kind of thing.
As you might be able to tell, ours has no single theme. I like for them to have a little bit of everything. I'm still looking for a nice trunk to keep it all in. For now, my daughter's outfits live in her large cauldron, and we keep the boys' in a small superhero pop-up hamper.
If you prefer to buy the costumes outright, Melissa & Doug have the best selection I've ever seen in their Role Play collection. Each set comes with accessories you don't often find included with costumes, nor in retail dress up trunks. For example, these adorable cowboy outfits come with the hats, bandanas, and sheriff stars: