Curate a themed series of sustainability-related activites to commemorate Earth Day, over a period of a week or month. This can be a combination of talks, guest lectures, workshops, activations - anything that suits your selected theme. The theme will be related to sustainability, and possibly one that's practical to apply to your daily life at work, such as plastic waste, food waste, learning about sustainable fashion, and more.
For example, if you choose to do a themed series on Plastic Waste, you could organise a plastic waste collection drive, upcycling plastics workshop, and a talk on plastic waste in Singapore.