My DD (5 years old) is going through a ballet phase, so she's all about the leotards and tutus. I've seen super-easy tutu instructions, involving a thick elastic and strips of tulle. DD would LOVE something like that in a sparkly or rainbow tulle.
Both my DD and DS (3.5 years old) have gotten a lot of mileage out of the generic princess dresses we have, too. There's a pink one and a yellow one they like. We have a long magician's robe, too, that they both like to wear from time to time. Ooh, and magic wands. You could make really cool ones from skinny dowels, glitter, a styrofoam ball, and other accessories. We've done that as a craft project from time to time. And crowns are great, too. I've seen some awesome felt ones; we've made them ourselves out of sheets of foam, decorative brads, and sticker, and they've been fun.
We also have superhero capes, with their initial on the back. We bought them on etsy; there are a bunch there you could browse for ideas. For awhile my kids were all about the superhero play, but now that's tapered off.
Fancy shoes are also a big hit, although if you are going to sew something, that would be much more difficult.
Whatever you give, I'm sure the kids will appreciate it; my kids would love forever anyone who gave them more dress-up clothes!
