Another brilliant idea I snatched from playgroup.
What you need:
Bit of firm foam
A handful of golf tees
A kid-styled hammer
Kids can use their hammer to 'nail in' the golf tees in the foam. This is a great activity- full of fun and great for developing those fine motor skills.
And of course, be careful with game and use common sense. If the piece of foam has deterioated badly then chuck it out and get a new one. And this isn't a game for babies but for mature children.