Maybe I just missed it, but I'm really suprised no one has mentioned peanut butter and banana sandwiches yet. Those are so great-I ate many more of those growing up than I did PBJ ones. I also like peanut butter and honey sandwiches.
Somebody did mention that peanut butter and butter sandwiches were good but rich, but personally I find that just a really thin layer of margarine on the bread under the peanut butter keeps the peanut butter from sticking to the roof of my mouth. I don't remember where I read that, and when I did, it sounded weird, but I really really irrationally hate it when the peanut butter does that, so I tried it, and it really does work. Personally I don't taste any difference, but YMMV.