I hesitate to even call this a craft, because it's so simple it's almost silly. But at my daughter's watermelon-themed 1st birthday party last weekend, I got more compliments on these little cups than on any of the other crafty things I spent way more time on. It requires two things: clear plastic cups and a Sharpie. Done and done.
I just drew some seeds in a tear-drop-ish oval shape, clustered mostly towards the bottom of the cups, and then stacked them next to some cute paper straws in watermelon shades. My main concern was that the drawn seeds looked slightly like beetles...but they really fulfilled their watermelon potential when we filled the cups with red drinks. For the party, I set them alongside watermelon lemonade, but you could use them with any red-hued beverage - from Koolaid to sangria - for a cute summery detail. Finally, after being spit to the wayside for so many years, the seeds get their time in the spotlight!
And if you wanted to take it further (taking a theme overboard is kind of my MO)...you could take that Sharpie out and draw watermelon seeds on just about anything...