Your LO should be a skill, knowledge or understanding. You want them to use exciting vocabulary. The context for your Learning Objective is a riddle. If your learning objective is to write a riddle, the skills of using vocabulary is closed only to be used in that context. If it is a skill learning objective, you want them to use that skill in other genre. What sort of vocabulary do they need to be able to write a riddle? Can it be transfered into other genre? If so, then your learning objective is to use exciting vocabulary. It is important to get the learning objective right, otherwise your lesson turns into an activity with a product at the end of it, and the learning is second to all that.