The party will be at a park... and at that age, they will entertain themselves at the park pretty much.
You could have a treasure hunt... a pinata that can easily be cracked open by 2 year olds, bubbles, and just having balls around is fun.
You could also have a huge poster board too, with tempera paint... and have each child put their hand-print on it and the parent paint their name on it... as a "souvenir" for your child for their birthday memories... and then you could hang it on the wall at home. And of course you write the date of their birthday party on it and their ages, so your girls will have something cute to remember their party/birthday with years later.
good luck,
Susan