25 washers/dryers, 5 Hiero Walls, 10 rock Piles, 10 wooden barrels, 10 castle walls

The only useful prize appears to be the Homer Barbarian statue, but it's unique and isn't awarded if the player already has it. I'm still saving up castle walls and towers in the unlikely event that we're ever allowed to use them in Krustyland. I'll probably get bored of it soon and use the dig grid to make a message or pattern to display.