Baby food can be great. Another plus, if you have a sick bird you need to get something nutritious and soft down. Getting them used to having it as a treat now can make them more inclined to eat it when they're in chicken hospital and since it has such a long shelf life, it can be a part of your chicken first aid kit.
Another interesting one is dried flaked fish. I used flaked king salmon cat/dog treats as a protein boost when rearing my SLWs to support their quick growth and they absolutely loved it.