If you want something you can wash and save for next year, use fabric paint - dab a little paint on the grasses and press onto the material to make prints, or lightly pound freshly picked fall leaves onto the material. Make matching napkins by cutting up a pillow case or sheet into approximately 10x10 squares. Again, use pinking shears to cut the pieces and leave as is, or hem the edges. Don't throw it away. Cut it into pieces and trim the edges with lace, ribbon, etc.
Old towels make interesting placemats.
You can decorate them using fabric glue and colored felt, or sew on embellishments as you like. Old jackets - especially winter jackets made of thick material make interesting placemats. Sew a piece of wide ribbon around the edges for trimming.