It's not mail art yet but all it needs is an address and a cancelled stamp, so I posted it just for the inspiration. Although I would leave the three-dimensional flowers off, as they pose a risk to getting the envelope caught in the machinery.
40 Ideas for sensory bags. Sensory bags are cheap and easy to make and can be appropriate for a variety of ages from babies during tummy time to kindergarten students. Use them at home or put at a "science" center at school.