I would wear a summery, cool, woven, loosely flowing, sleeveless with wide straps, possibly elasticized bodice, mostly white, maybe very faint pastel springlike print, cotton dress, and sandals with some simple bohemian earrings (especially with long hair), or with nicely tanned legs, short white cargo shorts and a loose fitting white cotton T with some kind of printing on the front a leather thong choker length necklace with a couple beads and some hip tan/buff strappy but comfortable sandals with a low heel.. :)

I would wear slacks or reasonable length shorts, not a skirt. Wear comfortable closed-toe shoes you don't mind getting dirty in case parts of the fair are on dirt or grass. You can wear a cute top but if there will be rides, don't wear one that will flip up etc. And if you may be there in the evening or get cold easy, consider carrying a lightweight jacket. I would wear sunglasses but make sure you have a place to stash them if you go on a ride.