Any of the green veggies, but only a little bit. It is not used to it and may scour. Corn, peas, broccoli, cauliflower, dandelion seed heads, in fact any seeding grass head, but again... a little bit at the time. Mix a little bit of canary seed in the millet, and perhaps a very little bit of oats.
Tie the veggies or seed heads to the top of the cage, it will force the bird into a bit of exercise. It should really have a daily fly around the room to stop it getting too fat, but that may be a bit too demanding for a lodger.
The standard cuttlefish bone, oystergrit and fresh water is always available ofcourse.