What are the ages of the kids?
You could pick a really hot day and do a "winter in the summer" theme and have them make clay snowmen and paper snowflakes, etc. Depending on the age may be they could try this snowflake?:http://www.wikihow.com/Make-a-3D-Paper-Snowflake
It's not as hard as it looks, I love making these!
They could decorate jars and use them as firefly/ladybug catchers?
You could buy a pack of seeds and decorate clay pots (or even egg cartons for a cheaper option) and then plant the seeds so everyone could take their own plant home, but depending on supplies that might break your budget a little.
I remember at the rec camp I went to in my town we once did Halloween In July too. Those things work well if the whole camp goes all out for a theme day.
Those are just a few ideas. Hope some of it helps and have fun!