Chickens tend to love marigold. It's perfectly safe for them to eat and can even give their yolks a richer orange color. Some chicken feeds have marigold in it for yolk color.
Lemongrass is safe. Lemon Balm is safe and has some antibacterial properties, and the scent has a calming effect.
Lavender is safe and also has a calming effect.
Catnip (and other mints) are safe and catnip has the added benefit of, you guessed it, calming effect!
All of these plants smell great (imo, some people don't like lavender for example because it is very strong), look great, and are beneficial for the chooks!
Just be aware of 2 things: mints are not native and spread like crazy and force out native plants. Plant them in containers to control the spread. And, if you let your chickens at the plants, they're
going to scratch and pick at them. Either let the plants grow and establish before letting the chickens at them, or have several plants ready that you can cycle out when one gets destroyed.