Dispensers with arrows (depends on mob drops)
Walls with lava in between (only one source block needed - vertically)
Pressure plates with pistons to push them over/in something
Spikes (in a pit) (depends on installed mods)
Electric fence (not really early game if I read your iron shortage correctly)
Or put a villager in a 2-deep hole somewhere and surround him with something mobs can see through, but not reach the man. Then watch them burn in the sunlight.
Decoys are a good idea. It only takes a light source over a pit.