With the Grill Grates and the roasting rack, you already have a couple of the best accessories for the grill.
The dome extension ring with the chicken roasting stand is one of the better accessories to have for the cooker. It will allow you to do some larger items like maybe a ham, beer butt chicken, turkey breast, pizza, bread, biscuits, and a few more items where you need to get what you are cooking raised in the cooker to prevent the bottom of the item to burn before the top would get done or browned etc. Two of these extensions are required for making a rotisserie setup for the grill which in my opinion is one great accessory to have on any backyard cooker.
Of course, the disposable inner sleeves (tin foil liners) are hard to do without as they make clean-up much easier. They can be washed.
The BBQ / Open Fire Grill Set with Wide Fire Grid is another nice option especially the larger charcoal basket. I find that I use that basket for about 75% of all my cooks. The cooking grid is nice in that it will allow some of the drippings from what you are grilling go down and into the coals for that additional flavor that comes with real bbq cooking. Your Grill Grate setup will also allow this as well in most cases. I also use this grid for my raised rotisserie cooking but I could live without it.
Ok, one or the other of these two accessories is nice to have. The top one is the Cobb Skillet or non-stick frying pan.
The second picture is the Deep Dish / Wok. I have both of these accessories and find that I can do about anything equally as good or close to it in either of these two accessories.
Fry an egg, make pancakes, French toast, fried cornbread, fried mush, fried veggies, and many other items are great in one of these two accessories. However, only one is good for soups, stew, and other treats with some liquid in them.
I guess if I had to choose only one, I would take the Deep Dish / Wok. It doesn't fry an egg as well as it is all Stainless and doesn't have the non-stick surface.
This accessory is not required if you own the Grill Grates. It is called the Cobb Griddle. I didn't own it before I got the Grill Grates because I thought it was way overpriced for what it does and what it costs.
I have another 6-8 accessories that a fellow can make with over the counter items that make cooking on this grill much better and more fun. Items like a 3 pound meat loaf pan with a removable insert, several muffin tins. a couple of cake pans, a second raised cooking grid made from a cooling rack, and others.