I tasted sunflower pesto at a nearby restaurant, Uncle Betty's, and it was fantastic! It adds a sunflower flavour that I thought worked very well with basil. I didn't miss the pine nuts at all. I spoke to the owner about it and he said made the switch to avoid nut allergies. Economical, tasty, and won't kill people by accident! I'd call that a win.
I'll try that myself another time, but I thought cilantro would make for another interesting substitution. I didn't change the rest of the recipe, but I think I should have. With cilantro, it came out a bit oily for my taste. It is much more of a liquid than the basil one was. I think the cilantro has more water in it than basil. Oh, and I added one less clove of garlic. Last time, the garlic flavour took over the next day. Hopefully it will tighten up overnight.