A couple things I've noticed in my limited career:
1. Seasonal events
like barbecues, holiday parties, picnics, potlucks, chili cook-offs and the like. Pick a friday of the month, tell everyone to plan ahead, and it will take care of itself. Nobody gets much done Friday afternoon anyway, so it isn't a big productivity loss.
2. Monthly meetings and awards.
One of my favorites on this was the award for the last person to arrive at the monthly meeting from Connexn. The last guy in the room received a trophy made out of a can of really bad beer. People who had done something good got a "GOBOSH (GO Big Or Stay Home). People who had been there for n*years got an anniversary certificate. People like awards. As Napoleon said "give me enough ribbon to cover the tunics of my soldiers and I will conquer the world." And when you give these awards - announce the person's name (learn how to pronounce it before hand), say "thanks", and look people in the eye.
3. Employee Appreciation dinner/breakfast/vacation
Some places spend lots of money on fancy dinners and think that's right - I don't think that makes much marginal difference beyond a hot dog from Mustard's Last Stand. Some places dont' do anything - that clearly won't work either. I like the idea of, first, doing things that help your employees get their work done and that keep them happy (like breakfast burritos on Friday morning once a month) and, second, that reward a common interest like a friend who works in a ski town and whose company flies employess who have worked there for 3+ years to Mexico for a vacation at the end of the year. All the employees take week long vacations in the spring/fall off-season anyway, that's part of the attraction of working in a mountain town. Letting them do it together builds corporate love.
Moral of the Happy Employee Story
These won't guarantee happy employees, but man if you don't do these things that's one sucky company.If these don't exist, start them.