I loved this idea for watermelon counting cards on the Confessions of a Homeschooler blog - but ran out of colored ink in my printer so opted to improvise. Here is my version (in which we used chocolate chips as our seeds instead):
I just made simple watermelon slices (seedless) of various sizes using construction paper. Then, I printed the numbers 1 through 10 on the back of each and laminated them (an essential step as we all know what happens when you combine chocolate chips and sweaty toddler hands).
One version of play is to flip all of the slices over so that only the numbers are exposed. One player chooses a number, and once they properly identify and say the number they can place the appropriate amount of "seeds" (chocolate chips) on the slice of watermelon.
Another way we played was with our over sized giant dice (found at the Dollar Tree - we have had so much fun with these). We would take turns rolling, count the number on the dice, and add the corresponding number of seeds to our slice.
Of course, in both games, you can eat all of your seeds once you've filled all of the slices!
I purposely made the slices varying sizes so that we could use them later for size sorting (with or without our seeds).