In my college days I once screwed a watermelon (y'know green outside, red inside).
Cut a hole in the top, not too big, so your semi-erect dick fits into it. Start pumping away, slowly. As you get harder, your dick will close up the hole, and the melon becomes a vacuum, creating a nice suction as you move in and out. It gets real noisy, slippery, and messy, haha!
I didn't cum inside the melon, but I had a lot of fun somehow. Try it if you like!