Love your mermaid by the way, she is just beautiful.
The first one that came to mind was the one arm row which you may already ge doing.
OK I'm gonna pull out my old link to the website with all the exercises.
This page has broken the back into three areas so you may want to look through them all. Some are with machines are some are free weights.
Sing out if you need anything else or if it doesn't make sense.