See my blog on fun questions to ask your boyfriend
Childhood inspires innocence and fun, so this is a particularly good place to start. 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. This is particularly easy because all you have to do is to remember a wonderful time you shared together as inspiration. For example, if you went to the park and she kept looking and touching the flowers, you can call her bumble bee.