1. A pen large enough for the number of chickens you have. Of course the bigger the better...but at least 4 square feet per chicken is the goal...although 8-10 feet per chicken is better.
2. A place for the chickens to roam outside the coop--a protected chicken run or a safe yard, etc .--a place where they can nibble and scratch and find juicy treats (bugs, slugs, weeds, etc.) to eat. (Hint for a happy chicken owner: If you value your garden, keep your chickens out of it...yes, they'll eat the weeds and bugs, but they'll also eat the leaves and vegetables)
3. Treats. Yes, they're just like us humans. Treats make us happy, right? Chickens are no different. Treats for them could be a worm or bug. It also could be fruits, vegetables, breads, pastas (cooked), or just about any table scraps you may have (there are some things chickens shouldn't eat). Most chickens love cracked corn (which you can get at the feed store when you buy chicken food), but don't overindulge them--too much of a good thing can sometimes be bad (I give my 10 chickens a small handful or so of cracked corn to SHARE. I throw it out on the ground and let them scratch for it--they LOVE it.)
4. Fresh water (you wouldn't want to drink poopy water, either, would you?)
5. Roosts. Make sure you have enough roost space for every chicken to have a spot. Roosting (hanging out on a pole suspended above the ground) makes them feel safe.
6. A chicken coop that is SAFE from predators, wind and rain. (Watch how I built my diy chicken coop.)
7. A calm environment (chickens are happier if they don't have a dog barking at them day and night, for instance).
8. Warmth in the winter, if it gets really cold.
9. Daylight (it takes about 14 hours of daylight to coax an egg out of your chicken).
10. A bit of dirt. Chickens love to scratch around in it. They give themselves 'dirt baths' to cool off in the summers and to help control bug infestations.
11. Be careful how you introduce new chickens to your existing flock. It can be a bit tricky, but learning ways to help the transition will go far in helping your chickens adjust.
Really, it doesn't take much to ensure you have happy chickens. Food, water, shelter, safety and an occasional treat...and your rewards will be fresh eggs, fresh fertilizer and plenty of reasons to laugh and smile as your chickens entertain you with their funny behaviors and great personalities!