No medications, no supplements, no vaccines, no worm meds.
Lots of space. That's the single most important thing in having a physically and mentally healthy flock. Overcrowded animals of any sort are more prone to physical illness and parasites. Overcrowded animals are mentally unhealthier also, and mental stress lessens physical resistance to disease and parasite infestation. Space is critical.
Beyond that. just basics. Shelter, water, food. A commercial feed as the base, with a nice variety of kitchen scraps to top it off.
I dump ashes from the woodstove in the coop or run for them to dustbathe in, helps keep parasites away.
I do try to vary the run from time to time. I don't think of it as "entertaining", them, I think of it as mentally stimulating them. Maybe that's just semantics, but to me no animal should be confined in unchanging surroundings day after day after day forever. No animal was designed for that. I just do things like move the hiding places around, flip over one of the stall mats in the run to uncover more bugs, add an old Christmas tree, move the waterers, things like that.
My theory is, if you're needing a lot of meds or wormers, etc, then you need to examine your basic flock management practices. Healthy animals shouldn't need those on a regular basis. To me keeping your flock healthy means preventative management, not chasing issues once they start.
Also, maintaining a closed flock or properly quarantining any older birds brought onto the property. Second to overcrowding, "rescuing" and bringing in swap/auction birds seems to be the biggest way to have an unhealthy flock.