Peter Quill: He made it look like he completely forgot and then when you left the Milano with Gamora, he got the guys to help him decorate and bring out all the presents he had hidden in the closet. When you came back, the Milano had paper lanterns of your favorite color hanging from the ceiling and Peter had made a mix tape of your favorite songs for you. You danced and laughed with your friends. It was a really good night.
Drax: You hadn't told anyone what day your birthday was, so you just made Peter land the Milano on a popular resort planet and spent all day at a hotel room with Drax. When dinner rolled around, you treated everybody to a really nice meal and afterwards played video games with your boys.
Rocket: Your birthday was coming up and you wanted to do something different, so a week before the big day, you started making a Bucket List of things you could do on your birthday. Rocket joined in and started listing off things he wanted to do with you. One of the things you wrote down was having a paint fight with the Guardians and Rocket made it happen. Peter bought fifty cans of different colors of paint and you guys had an all out war. You, Gamora, and Rocket versus Peter, Drax, and Groot. In the end, your team won and you all were covered from head to toe in paint. It was the best day of your life.
Groot: Everyone wanted to take you clubbing, but you stayed behind to watch over baby Groot. Spending the night with the person you love is how your birthday should be spent. And Groot would give you the greatest gift you could've asked for. While you were making a drink, you heard something like roots being ripped out and ran back to check on him. He wasn't in his pot and you found him running towards you. Barely taller than your calf, he hugged your leg and managed to say "Groot loves you". There were tears.
Yondu: He took you on a night out and after a wonderful date, he had one more trick up his sleeve. You had always told Yondu about how much you wanted a dog and he took you to his room and started whistling. The fluffiest collie puppy ever came running towards the both of you, crashing into your legs. You quickly forgot Yondu existed and cooed over the puppy. You named him Scout and Kraglin gave him a spiked collar to try to make the ball of fluff look a little tougher.
Ronan: From dawn to dusk, he gave you plenty of small gifts and even had a feast in honor of you. Every Kree on board the Dark Aster attended and even Nebula managed to act polite towards you. It was a little awkward being the center of attention, but after telling Ronan about how humans celebrated their birthdays, he was determined to give you a perfectly normal birthday experience.