Just buy one of the packaged sands. If you get aragonite sand, it will help keep your pH in balance. I'd want the sand to protect the rocks from scraping the glass and to give a more natural environment for my ceph - and also, it contains bacteria that will help your tank. If you use live sand, it's already wet and washed, so it's minimal effort.
They sell bags of Florida Crushed Coral, If you get the fine grit its almost like sand but its heavy enough to sink away from a gravel vac. Plus I think it may not be nessisary for your octo to have a substrate but the benifits are huge... exp. like Nancy said it helps balance the PH its home for 1st and 2nd stage benificial bacteria (NItrobacter & Nitrosomonus bacteria) and it makes the tank look 100% better.