For both DD1 and DD2, a local venue was hired. These venues chosen had both indoor and outdoor space (one a hotel function room, with a garden, the other a local cricket club).
The children traditionally dress up for their last day at the school, so all walked down in their fancy dress. A disco entertainer was hired for one hour, they had party food, and some of the heroic parents ran games like rounders and so on. Some of the girls had supersoakers.
They were provided with cheap white tshirts for all their friends to sign, and both my DDs still have theirs.
There were cupcakes for DD1's lot (they were right at their peak of fashion) and a fantastic decorated cake for DD2's lot, which had all their names on it, so they got to take home the bit with their name.
We were very lucky to have fabulous weather both times, but we obviously had indoor options, as well.
I can't remember what the cost was for DD1, but for DD2, it was £15 each.