Does your rice cooker have a steamer basket you can put above the rice? If so, you can put some cabbage up there to steam up while the rice cooks. If you eat meat, you can also put things like pre-cooked sausages up there. I was pretty partial to putting andouille sausage up there and I made Jambalaya in the rice cooker pot with sliced okra. While steaming the andouille flavors would drip down into the Jambalaya, adding even more flavor to the rice. Also, no further dishes to clean. One pot meal.

Edit: Oh I totally forgot, you can steam onions up there too, but you may want to soak the sliced onion in a bowl of cold water for five-ten minutes beforehand to take some of the bitterness out as they won't cook sufficiently to sweeten.