Me and my boyfriend didn't want to have to carry around a big bottle-type mister fan every day at the parks, so we were looking for a small one. We found these last year at Wal-Mart:
http://www.campmor.com/outdoor/gear..._medium=CSE&CS_010=47183&utm_source=Shopzilla
We just clipped them onto our backpacks and filled them up every now and then at water fountains. The battery cover on mine did fall off a day or two into the trip, so if you got one, I'd reccomend taping it on. They're pretty powerful for their size! And they're super cheap, too.
I've also seen them at Academy Sports stores.
We used these for our trip last May and loved them! My DH is always warm (shorts and t in the Winter type)...this little fan helped keep him cool despite the hot May temps and blazing sun. We also filled them up at the water fountains.