Read about things to ask your girlfriend
Now this is easy because you will simply have to remember a particularly lovely time together and take inspiration from there. For example, if you went to the park and she kept looking and touching the flowers, you can call her bumble bee. Another good source of inspiration and the most commonly used ones is food. You can choose from sweets, like cupcake, or you can choose from fruits, such as strawberry. She will surely appreciate it that you think of her as something as cute and sweet as the fruits and sweets you use.