Today is my birthday, I've had at least forty of them, so I consider myself a budding expert in the field. My best advice is if someone is having one, wish them a happy birthday, whether it's a clerk in a store, your best friend or second cousin on your mom's side.
Birthdays are the days that our stories began. It's good to remind people that you're thankful to witness a piece of that story. If you have co-workers, bring them their favorite dessert. Doesn't mean cake every week and it doesn't have to be for sharing, could just be a slice of pie. Small gestures are better.
Some folks love big huge parties, introverts I've noticed fall on the other end of the spectrum. You're not likely to know about their birthday unless they want you to. So take it as a compliment when your introverted friends invite you in to their world. Repay the favor by following their requests when it comes to gifts. If we say your company is the gift, believe us. Awkward gift opening is not our forte. Well, I guess there could be a few introverts gifted in that field. But they're likely ambiverts in disguise.
Birthdays are really just an excuse to celebrate people. Do that with wild abandon! Wish people well, speak into their lives and spread love. Tell people things about themselves they may not see, encourage them in the rough spots, be genuine in your affection towards them. We don't get everyday with some folks, but speaking a blessing over someone on their birthday is not only appropriate, but well received 9 times out of 10. That tenth time, you've waited too late in the day and it's probably best if you just continue to hold their hair while they puke instead of having a deep conversation.
Birthdays are a natural pause in the year to tell people you see them, love them and appreciate their story.