These ideas are short and sweet: I simply dug into our box of dress-up clothes. My kids are young enough still that they didn't care that I didn't go out and buy them a special costume. Hopefully they never will care. The point is not to spend money, it's just to dress up, right?
For the Dora costume, my daughter had gotten this weird little Dora mask/hood with a cape attached to it for her previous birthday. So, to go along with the face, I headed off to Good Will, my favorite Halloween costume shopping place, and found a pink T-shirt and pair of orange shorts. She threw on a backpack, and "Super Dora, to the rescue!"
The Tinkerbell costume actually WAS a costume we'd found at Kmart the year before, AFTER Halloween. All their Halloween stuff was 75% off in November, so we stocked up on fun stuff like wigs and a couple costumes for the kids to play with. I didn't mean for it to be an actual Halloween costume, until the next year, I said "Hey, how 'bout being Tinkerbell for Halloween?" She loved the idea, and ever since then she's been rather obsessed with Tinkerbell. (This is our baby pirate from the previous post - how big she's gotten!)