One I like is peanut butter and apple. A sweet, crunchy and juicy apple is best – Pink Ladies work well.
More often, though, I’ll just go with peanut butter and Marmite, because it’s always in the cupboard.
Many years ago, peanut butter and onion sandwiches were a thing in my life. Haven’t had one in years, but I do recall enjoying them.
Using yellow, red and green onions in rotation, I found the green onions, chopped, were my favorite.
I was just going to ask about this! Made me wonder also about peanut butter and miso, since Marmite and miso have a similar flavor profile. The miso would be spread very thin as I assume you’re spreading your Marmite. I don’t have Marmite on hand but I do have miso. I’ll try that and report back.
Another vote for PB and honey, with sliced banana.
So I rarely have sliced bread in the house, probably because I’d have too many sandwiches!
(I kid, my wife would finish the loaf and peanut butter long before I overindulged 
)
For me, it’s got to be a decent whole wheat bread (so, a minimum of the Kroger Private selection bread), preferably the crushed wheat variant.
Lightly toasted, and left to cool for juuuuust long enough for the peanut butter to not melt, but not long enough for the bread to get stiff.
Peanut butter is generally Trader Joe’s creamy, or an equivalent made with just peanuts and salt - stirred vigorously to make sure it hasn’t separated.
Pretty thin coverage, but complete cover.
Now the jam - I’m rarely in the mood for jam, but if I am, it’s normally a quality raspberry (the wife is all strawberry, and she especially hates seeded jams). My preference is for a “hot” honey, which is honey seasoned (very lightly) with habanero and cinnamon. Super tasty, but a little goes a long way, and I don’t want it to overpower the peanut butter and toast.
[ ETA - there are plenty of hot honey variants out there, often cheaper than the one I linked, which just happens to be the one I remember using last ]
Another variant is the above, but instead of honey, a thin drizzle of the best, darkest maple syrup you have on hand. 
Or, you can just throw health to the wind and go straight to cookies, like this:
A local restaurant is known for its burger with peanut butter (no jelly). I had one, and I gotta say, it was much better than I anticipated.
My own hack is to drizzle just a bit of maple syrup on the PB sandwich. Another successful experiment was to sprinkle granola on the sandwich.
And honestly, I am fine with simply eating peanut butter off of a spoon.
mmm
This time of year the nearby farm store has all kinds of apples. I’m working on dinner preparation now and my dessert will be baked apples. I will be stuffing the cores with peanut butter.
As a kid, my mom always gave us apple slices with either peanut butter or honey.
Booo! The PB has to be slathered as nearly as thick as the bread it’s on!
I salute the power of your palate! But for me, I get kinda uncomfortable with the peanut butter sticking to said palate - I don’t feel like I’m going to choke, but it isn’t pleasant. So thin it is for me.
Which is why such a thread is all about tuning a food to your needs and preferences!
It’s been ages but I used to love PB, banana and cream cheese sandwiches.
PB and mayo on crappy white bread. My older brother introduced me to them when I was a kid and I still have one from time to time.
Something I’ve pushed on this board in the past is a hot dog topped with crunchy PB and then a drizzle of sriracha. Smoky, salty, sweet, crunchy and spicy.
So many things are delicious additions to a PB sandwich - I am in favor of many of the suggestions here. A couple more:
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Apricot jam is a remarkably tasty jam for a PB and J.
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My mother used to make a sandwich filling of equal parts PB, cottage cheese, and raisins. It looked horrible, but tasted good!
We have a cookbook called something like Victory Garden, from World War II. It has these recipes for peanut butter sandwiches. I have not tried them.
PEANUT BUTTER AND PICKLE SANDWICHES
1½ cup peanut butter
1/4 cup cream or hot water
1/2 cup chopped pickle
Cream peanut butter and water together and add chopped pickle. Use between thin, buttered slices of bread.
PEANUT BUTTER AND ONION SANDWICHES
1 cup peanut butter
¼ cup mayonnaise
1 small Bermuda or Spanish onion
Beat peanut butter, add mayonnaise and spread sandwiches. Slice onion in very thin slices and put a layer of these over mixture on bread.
Of course the most famous peanut butter sandwich is probably the Fool’s Gold Loaf, that was made famous when Elvis flew from Memphis to Denver just to get one. The original restaurant that made it (Colorado Mining Company, I think) is long gone, and last I heard the place that carried on the tradition for many years after has recently closed (Nick’s Cafe).
It is a hollowed out French bread, filled with a pound of bacon, a jar of peanut butter, and grape jelly, then deep fried.
Wow.
That last entry demands a moment of respectful silence.
I was introduced to PB&DP sandwiches by my SO, who got it from one of her friends. Just peanut butter and dill pickle slices (or dill relish) between two slices of bread.
I don’t do it often but sometimes I’ll add a bit of green olive tapenade from Harris Teeter to my PB&J. I don’t remember what prompted me to try that but the saltiness of the tapenade really works with the sweetness of the jelly.
That’s my favourite sandwich. Two slices of bread, crunchy PB, and half of a banana, cut lengthwise.
Concur.
Here you go, a 9-way PB&J:
I see that the “fluffernutter” has not yet been mentioned. (I know, I know … the rude jokes almost write themselves).
That awesome concoction has a filling of peanut butter and marshmallow creme, which at a bare minimum needs to be quickly spread between two slices of toast while they are still hot, to promote melting. Alternatively, the sandwich can be made with bread, the outsides of which are then toasted under a broiler. (I guess you could also grill the sandwich, but that sounds too greasy to me.)
How I loved fluffernutters as a child!
Yeah, baby! Marshmallow fluff! Now that’s the thing to eat out of the jar by the spoonful.
My son invented PB with pickles. Sounds odd to me, but he likes them. I love PB smeared on a banana.