A hall is a big space to fill. At church hall parties we've been to there have been 2 slides, small climbing apparatus, about ten ride on vehicles, mini trampoline, large play tent, toy kitchen play area, large boxes of vehicles, figurines, dressing up box and so on.
We've been to one where someone hired a bouncy castle and soft play equipment but that will have been pricey.
If the weather is good then there's more scope with the outdoor space, but again what will there be to do? Last summer we went to a pool party which was fab. The family had a very large inflatable pool, big enough for 10 two year olds to play in, as well as a bubble machine and lots of garden toys.
Cheap way to use the outdoor space would be to borrow a gazebo from someone, bring lots of picnic blankets, food and teddy bears and make it a teddy bears picnic. Tell all the children to bring their favourite bear. Have party hats for all the bears, few bubble machines, as many toys as you can bring, maybe a treasure hunt type game or something like that, music and good food for everyone. You could have a little station where the party hats are with lots of stickers and the kids can decorate their own party hats for their teddies.
If my child had a summer birthday I would absolutely do this.