I let the kids vote on my costume from the books we have read so far, and they chose Leo from Leo the Late Bloomer.
I think Miss Frizzle is great if you have read any Magic School Bus books. Cat in the Hat is also pretty easy...
find some sort of theme dress (fish, planets, shapes, etc.) at a thrift store then build around that... then add colored heels and a red wig!
I'm going as Pinkalicious! It works if you own (or can borrow) a bunch of pink clothes, then all you need to buy is a pink wig. I happen to own pink tights, pink skirt, pink sweater, pink shoes, so it's an easy costume for me!
One year I was the 'cap seller' from Caps for Sale. I wore one of my husband's suit jackets, a stick on mustache, stopped by the thrift shop and bought a bunch of caps and the children colored and cut easy monkey-face masks. It looked cute. Also was the Queen of Heart-wore all red with a crown and scepter, and made an easy sandwich-style queen of hearts playing card. How about Fancy Nancy? She's easy.
A teacher at my school was captain underpants last year and it was hilarious. She just wore a big pair of white underwear over her clothes, and made a cape.
I was going to suggest Cat in the Hat or Thing 1 or 2. Although I love some of the ideas on here already! My kindergarten son loves Leo the Late Bloomer, he would totally dig that costume.
How about Fancy Nancy? My 4 year old is going as her this year! We have a fancy dress, bright pink boa, lots and lots of bead necklaces, sparkle shoes, big earrings . . . very cute. We found our accessories at the local craft stores. You can use all of her big word too like "extraordinary" "spectacular" etc. Love her! and I am so glad my daughter chose her! It would also be fun to be Pinkalicious or one of a fairy from Fairy Houses!