Bags of dogfood. Bags of kitty litter (oil absorbent). Suitcases or duffel bags loaded with cast iron weights. Bags of water. Cases of bottled water. Bags of steel or lead shot. I shy away from flammables like cases of motor oil.
More work, but if you're up for it and have a truck or trailer, buy 1,000 lbs of play sand at the local bulk landscape supplier and bag it yourself. It is cheap that way.
All of these things should be properly secured.