I used hydroton (clay pebbles) with great success. It aerates and improves drainage (not that coco doesn't have great drainage by itself) just like perlite, but it's heavier. It stays in place in the coco, doesn't sink or float. It's not as cheap as perlite, but it doesn't degrade easily and can be reused.
I'm currently using perlite, but I'd use hydroton again. I'm thinking about ordering it for at least one or two DWC buckets.