What is a good way to lure animals / mobs into big holes/traps?
They still seem to like to swim, so you could make some Water Traps - or any running water, maybe combined with a waterfall, into your pit.
Not very cheap, but it's a good tactic: combine some pressure plates together with dispensers filled with arrows or snowballs.
That way, whenever mobs get near the hole, the dispenser will fire at the mob, hopefully pushing it into the hole.