You know the story about using all parts of the buffalo...well, we did that with our watermelon:
First: dehydrated watermelon slices. This is pretty energy intensive and impractical since watermelon is 92% water, but the dried watermelon slices make very sweet fruit leathers.
Next, I pickled the watermelon rind. I just cut off the dark green skin and soaked in salt water (1/4 cup salt per quart of water) overnight. They taste like salty cucumbers--really good! I used the recipes from Dave's Garden and pick your own dot org.
Finally, we roasted the seeds that I picked out.
P.S. Best watermelon recipe I've ever had.