Here are some inexpensive craft items that my son has used to make a sticky wall collage:
feathers, pom poms, tissue paper squares, pipe cleaners, stickers, foam shapes / letters / numbers / animals, ribbon pieces, buttons, craft sticks
Even try clear contact paper taped to a glass door on a sunny day for a very colorful collage result. My toddler just loved how each item stuck to the paper. This is such an easy and exciting project for toddlers to enjoy.