I have only experimented with this on Logan bread (a dense bread designed for hiking/backpacking). That said, here are things I've tried:
Dried fruit (dates, apricots, cranberries, apples, etc.) - good to excellent
Nuts - good
Then we get to the more interesting ones:
Nettles - makes for a somewhat green bread, and presumably adds protein. I don't think I've ever added
enough to taste them. It's simplest to add dried
nettles to wheat berries when you're grinding your own flour.
Lambsquarters - my brother tried this. I don't recall it very well, but I think it was similar to the nettles.
Hopi red dye amaranth - we were hoping to get red bread, but it didn't change the color that much, and I don't think we added enough to taste it.
Chia - absorbs a lot of water. I add it for extra nutrition and
energy, since I eat Logan bread mainly when I'm hiking.
Peppermint - finally, this one's excellent! I think I've added fresh peppermint when grinding flour, which makes the flour stick together a little, but works fine. I've added nettles several times, but peppermint is good enough to make it a standard addition.
I would imagine you could add any herb you liked, fresh or dry, like I do with the peppermint. Anyone tried something like a tomato-basil bread with sun-dried tomatoes? Suddenly I want to experiment....