Is it tactile? Kids like using their hands to manipulate different types of materials with different textures. Exploring how things feel, fit together and come apart is an exploratory adventure already. Collage, sculpture and fun assembly projects make this easy. Best part is no matter the theme, you know that your students are working towards developing those important fine and gross motor skills.
Is it sensory? A sensory environment can be set up in different ways. They can be found objects in nature, plastic items from the craft store or whatever recycled trinkets you've set apart. Playing with these types of materials are great for children's development. It allows them to explore and investigate in new ways.
Is it open-ended? Children generally love when they have an opportunity to play and explore freely. When using open-ended materials the activity goes from average to having a high potential for supporting creative thinking. Allow your children to explore materials as they see fit.
Hopefully these three questions will help you in the journey of art exploration!