From Just Food:

Pickled Watermelon Rinds
Rinds of 1 small watermelon
1 qt vinegar
1 qt water
2 pounds sugar
1 tsp. whole cloves
1 tsp whole allspice
3-4 cinnamon sticks
1 lemon sliced thin

Soak the rinds overnight in salt water (1/4 cup salt to 1 qt. water). Drain off the brine and cook the rind in fresh water until tender. Drain. Put the spices in cheesecloth and tie up so that it is a little spice bag. In a pot on the stove, combine the water, vinegar, sugar, lemon, and spices. When the mixture is hot, add the rinds and boil until the rind is clear and the syrup is a little thick. Put in sterilized jars and seal.