This is a good topic. I have found that keets learn to eat new things from each other. My first keet wouldn't eat any veggies. Then I got her a playmate, and they both developed the confidence to try new foods. My newest boy ate the veggies on his first day with us, I think because he saw the older birds do it. My keet's favorites are:
chopped broccoli florets
flat leaf parsley stems and leaves
whole strawberries (they like to pick off the seeds)
any kind of lettuce/spinach
shredded wheat biscuits (no sugar or salt added)
quartered hard boiled eggs
To get them to eat the pellets, I put them on their veggie dish and they eat them readily. I think they associate the veggie dish with good food, so they try whatever I put on it.