The thing that surprised me when I started calling was the difference between people who go Ceilidhs or barn dances regularly, and people who go once to a PTA event, or party.
I already knew the difference between Ceilidh dancers and people who did Playford or went to 'Dancers dances'.

At a party they are not necessarily there to dance, they may not want to listen to long explanations. So it helps to have some very simple dances up your sleeve (Galopede, Blaydon Races, Virginia Reel, Big Set, a silly clapping dance like Mr. Porter). It also helps if you've got a radio mike and can go down on the floor and prance around (or move people into the right place). With lots of children it can help if you've got some dances that don't need a partner (Hokey Cokey, Conga, Horses Brawl, Snake dance).

One that I got from the Trad dance callers list is Blobs. I've not tried it yet, it would need the right shape dance floor and the right crowd.
You ask them to line up in groups, one blob below the next.

Right and left hand star
Circle left and right
All except the top blob open the circle to make 2 lines, and move up as the top blob moves down to the bottom
Lines forward and back twice

This might be done in 32 bars if the blobs were small and quick, but since you only have one set you can call at a speed to suit them (trying to keep some sort of phrasing, but maybe giving 8 bars instead of the usual 4 for each circle or star).