Bring living things into the classroom. Class pets like fish or rabbits can be decorative as well as educational. Just be sure you have the time and resources to care for them properly. If you're not allowed to have animals, plants and flowers create a soothing environment and help improve air quality.
Choose decorations that correspond with your subject matter. For example, if you're doing a unit on marine biology, hang plastic fish from the ceiling, decorate with pictures of marine wildlife, and create a bulletin board with a diorama of the ocean floor. Changing decorations often helps keep students interested.
Invite your students to participate. Helping to choose the room decorations will give the students a sense of involvement. For example, have each student choose a rainforest animal, create a life-sized paper version, and cover a wall with the creatures in their natural habitat.