Spray them with pond foam and then carve into natural looking shapes. Dont forget to cover the openings a little so you can carve them into irregular shapes. After you carve the foam, you can silicone them and roll them in sand or gravel etc. I think it would look best if you spaced them somewhat randomly.
Torch em!
Seriously. When you burn white pvc it bubbles and turns all shades of brown and black. With the PVC hot and bubbly, .....