Mix and MatchCut out two of various circus shapes (clowns, lions, elephants, etc.) Put them all in a bowl or a clowns hat and let the children match them and then sort them any way they see fit (animals/ non animals, by colors, etc.)
Balloon MatchMake several sets of balloon pictures. Let your children match them by shape, color, size, or anything else they want to.
Balloons and BlowingGet either kazoos or the party favors that open when you blow out on them and show the kids that the same thing happens when you put the end of a balloon over the part you blow in to. Younger children will be amazed by this!
Counting Circus AnimalsGet a large circus scene on a poster or collage that you make and let the children count how many (clowns, tigers, etc.) they see.
Balloons and WeightTalk about how different gasses weigh differently, then bring in two balloons one blown up by you and one by a helium machine and let the children see the diffidence. This is even more impressive if you can get a helium machine and blow it up before their eyes!