Revenge on girlfriends and exploring the topic
This is a good place to start since childhood particularly inspires fun and innocence. Toys, cartoon characters you used to watch and other childhood fun stuff are where you can find such cute names. Another category is the memorable times category. Now this is easy because you will simply have to remember a particularly lovely time together and take inspiration from there. For instance, you can choose to call her bumble bee if you went to the park and she kept looking and touching the flowers.