maybe hire a jumping castle
all day fun

we do treasure hunts like follow the arrows etc.

what about a paint/craft table did this for my sons 6th birthday was a blast.
gave each kid a canvas from the reject shop and let them loose with some paints glue glitter feathers etc.
just covered a table in an el cheapo table cloth the plastic kind.
was a blast lasted a good hour for that one and they could come back to it