Here is a great list of Instant Pot Accessories that includes bowls (for pot-in-pot cooking), pans, lids, lifters, steamers, etc. You'll find more on the Instant Pot Facebook group also.
My most used accessory is my steamer (the kids call it a spaceship). I use it on the bottom over water if I'm steaming something or I throw it on top of just about any meal to add some mashed potatoes and carrots for a side dish.