I almost always substitute walnuts.
I am deathly allergic to pine nuts. I sub walnuts, almonds, pistachio, filberts, peanuts for pine nuts -- they all work. Just don't offer me pesto with pine nuts!
I usually go with almonds or walnuts.
I never thought of peanuts, do you think a dollop of peanut butter would be work?
i don't like too much stuff in my pesto. the kind i made today just had home grown basil, olive oil, pepper and garlic.