I use a metal chopstick to stir my PB (after pouring off a bit of the oil). It's a bit smaller than a butter knife, which means it can glide through the PB without risking spilling. But honestly what makes the most difference for me is that when I'm serving PB I use the knife to reach all the way down to the bottom. Otherwise it ends up getting pretty hard down there, even if it was initially very well mixed.
Also, I'll there's a lid-based mixer [1, scroll down a bit] that apparently works well, but kind of looks like a mess to clean up. I believe that it is very effective though — much better at getting down to the bottom than any non-invasive solution.
I’ve had one of those. They aren’t nearly as mess-free as they claim to be. The lids don’t quite fit right over the jars I get here in Canada, and I always get a bit of oil spillage.
Since we started buying the Farm Boy 500g Organic Peanut butter[1], I haven't had an issue. It’s a good size, lasts a few weeks (my wife has some every day; I have some periodically), and is quickly stirred well on opening with a simple butter knife without spilled oil.
Also, I'll there's a lid-based mixer [1, scroll down a bit] that apparently works well, but kind of looks like a mess to clean up. I believe that it is very effective though — much better at getting down to the bottom than any non-invasive solution.
1: https://www.nytimes.com/wirecutter/reviews/best-creamy-peanu...