treasure hunt with just stickers or tattoos for the prize.
balls and tossing them into a box decorated like an alligator or something.
decorating paper bags
pinata with non-candy stuff in it
stamping on a HUGE banner size of butcher paper
making Christmas ornaments with paper plates and glitter and glue
Just go to your local Dollar store and pick up some stuff... doesn't have to be expensive.
*with board games... the pieces are liable to get lost... and if it is your own kid's board games... they might not like that the pieces are "lost."
Making sure that your own kids, do NOT feel they have to "share" everything and all their toys or "special" toys. I would... prior to the party... designate clearly, WITH your kids, what toys or personal items they absolutely will not or do not want to "share" with all those kids. It should be "their" decision (with your guidance), BECAUSE- things can break or get lost or just never be found again. Their things. Kids feel better in their own home... when they know that they do not have to give/share everything with outsiders/other guests/kids. And it is perfectly fine. It is their possessions after all, and at that age, some toys/things are just special. I would also cordon off their own bedrooms/private areas as well. Unless your kids don't mind, nor you.
Just a thought.
Is it just 1 Sitter you are having? How many kids per Sitter or what is the ratio of Kids to Sitter? Maybe 2 sitters is best if you have lots of kids... because doing crafts/activities/games with that range of ages... just "1" Sitter might go crazy containing them and/or any tiffs or tantrums. It would be a handful for even just 1 parent.
All the best,