Our most often used accessory for our Rad Wagon has been a crate with holes. It holds well with just two or three bungee cords and still allows a passenger on the back. Typically we put a tarp underneath to protect the wood deck which could also serve as a bike cover in an emergency. In one of the pictures below we managed to find a cooler that fits perfectly into the crate which we use for grocery shopping or packing a picnic lunch. When we're really hauling a lot, we use a backpack and a messenger bag in addition to the crate which sit nicely on the running boards (no need real pannier bags, at least for us). We use a rope to attach the handles which holds them in place.
Curious to what others use with their Rad Wagon.