I'm a big fan of peanut butter with slices of ripe banana. Preferably that banana is right on the edge of being overly-ripe so that it really smooshes into the peanut butter. Yup, that's heaven right there!
I've recently come across several mentions of putting slices of fried bacon on peanut butter sandwiches. While this does seem to be a bit...gratuitous, I could also actually see the salty bacon and salty peanut butter going together quite well. Maybe?
What do you like on your peanut butter sandwich?