For us, one surprisingly big part is... manners. Really. Being genuinely thankful when DH hands me the tv remote or when I hand him dinner. It's the same for the kids. A thank you when you open a door or pass the salt. I'm surprised at how much of a nice, cheerful feeling it brings. It was quite noticeable when I stayed with a friend and they all just barked orders at each other or never said thank you because it simply wasn't expected. The atmosphere was almost grumpy.
DH and I love to spend time together. We try to always compliment each other randomly, and we often walk up and give the other a snuggle as they're doing the washing up or making a cuppa. This wasn't always the case. We were just... normal years ago. However, once I realised that we didn't feel close, I showing physical affection a lot, DH did too and it actually brought us closer.
We watch the same tv shows (even if it's more mine or his kind of thing)
We do the shopping together because it's easier. We share household tasks when we can. When I know DH is off to change the beds I go help because it takes seconds with two working as a team. He does the same when I'm changing the beds. (Though sometimes one will sneak away and do it to surprise the other with a nice fresh bed that night.) Or if One of us goes to empty the kitchen bin, the other will pop the new bag in or take the full bag to the wheelie bin.
It's a happy, harmonious house where DH and I move in sync, like those cheesy tv ads where the couple are throwing plates and food to each other making a meal like a dance.
Our happy marriage doesn't feel like hard work. Being happy feels normal and effortless, even after 16 years.