Creative coding often involves exploratory programming to produce digital artwork. Artists typically start with a high-level semantic goal like a "stained glass filter" and then vary code parameters like shape, color, lines, and opacity to achieve visually pleasing results. However, translating semantic constructs to code syntax can be effortful. Current programming tools also do not facilitate rapid creative exploration. To address these challenges, we present Spellburst, a large language model powered creative coding environment. Spellburst provides: (1) a node-based interface for creating generative art and exploring variations through branching/merging, (2) expressive prompt-based interactions for semantic programming, and (3) dynamic prompt-driven interfaces and direct code editing to seamlessly switch between semantic and syntactic exploration. Evaluations with artists show Spellburst's potential to enhance creative coding and inform the design of computational creativity tools that bridge semantic and syntactic spaces.