Ohhh hun with DD1's 2nd birthday in 2 wks i was wondering the same, i have decided no SET games BUT i just picked up a 10m role of butchers paper from the reject shop for $4 and woolworths had little packets of pencils for 50c a while back SOOOO i am going to role it out on the ground and if they want to draw etc they can... you could keep it as a 'mural' of works from his friends/relatives on his birthday.
I have a spare canvas here, i was almost thinking of getting hand print of all the kids that come, writing their name next to it and letting them plop some paint down too... hehe

If you don't have any pick up some cheap balls (tennis balls, bouncy balls) and just let them run wild with balls, bikes and toys!