You can try:
unskinned cooked peas: 10 mins in boiling water, remove the shell.
Raw cucumber: Slice cucumber in half and microwave for about 30 secs to soften it.
Spinach leaves: Cook leaves in boiling water about 10 mins and freeze for about 30 mins.
Raw carrots: i heard of others feeding fish this but i could never get mine to go for it.