Send cards with small, flat things found at a park:
Spring / summer: blossoms, flowers, grasses, green leaves,…
Autumn / winter: coloured leaves, berries, twigs (with buds or needles), bark, roots,…
Message inside: »Greetings from your local park.« + information about the town’s parks
Give-away at DIY-stores and garden centres; also possibly Direct Mail:
Seeds of trees, bushes, grasses… in a plastic »Ziplock«-bag. Staple to a map of the nearest park (or a leaflet with additional information)