You can set up a series of spawn points above that are used randomly, every x seconds pick random spawn point instance or you can create objects at a random range of X across Y every X seconds. Lots of ways to do it. The objects will need some other logic set up to move down, or you can apply physics behaviour.
So the logic will look something like this:
Every X seconds
Pick random SpawnPoint -- Spawn Obstacle at SpawnPoint
Every X seconds -- Create object Obstacle at X=random(0,1000), Y=0 (co-ords are flexible)