This story is based on a suggestion from Ray_Writes: And maybe, instead of something big and extravagant and commercial, the Doctor takes her to some place (be it past or future) where they celebrate Christmas (or some sort of solstice festival if they haven't adopted Christmas as a holiday) in a really simplistic, meaningful way.
I'm not sure if what I wrote is anything like what you had in mind or not, so there may be another fic based on this prompt coming along in the future if you want me to try again. :)
Donna looked around with interest as she followed the Doctor down a dirt road that passed through the middle of a small, primitive village. Wood and canvas stalls were set up on either side of the road, and the bright green people inside of them were selling wares and food.
"The people here celebrate by giving special blessings to the men and women," the Doctor explained as they walked hand-in-hand. "They believe Christmas is about new life and building big families. It's been that way for hundreds of years here after war devastated the planet. Repopulation has been slow despite their efforts, but eventually they will spread across the universe."
Donna nodded as she took it all in. This was a far cry from the way people celebrated Christmas in her time, but it sounded interesting. The village was small but they held a ceremony each year, during which the village elder blessed the people of the town.
They joined the small group of people who were waiting for the ceremony to start. A circle had been made in the red dirt, with two wooden thrones in the middle and a hut off to one side. Lush forests surrounded the village, and the trees provided a dark purple background for the ceremony. The crowd was mostly made up of locals, but a quick glance around proved that there were a handful of other visitors present.
The sky was fading into darkness as the ceremony finally began. The elder walked out of the hut first. He was an intimidating humanoid man with dark green skin, large gills on his cheeks, and webbed fingers. He took one chair, and then his wife joined him in the other chair. She was obviously with child, her middle straining the simple brown dress that she wore.
"It is with great pride that I welcome you to our annual Christmas fertility festival," the elder said, and waved his hands around in a clearly deliberate way. He looked terribly old, with wrinkly skin and a hunched over back.
"Fertility festival?" Donna asked under her breath.
The Doctor leaned to whisper in her ear. "Like I said, they want to rebuild their civilization, so they've twisted Christmas to fit their needs. A bit like your lot, really. Did you know that Christmas originated from pagan winter solstice festivals-"
Donna tuned out the oncoming ramble and focused on the elder, not wanting to miss anything. He now had one of his webbed hands on his wife's bulging middle. "Cerah is carrying my twentieth child. No other man on Sertfe is so blessed. This is because I am the supreme male, and I bestow my blessing upon you all today. Our population has grown by one hundred this year. We must do better in the coming year. If you are of age but have not yet taken a husband or wife, you must be ashamed. You may not attend the feast, but must go home and hide your face."
"But harsh, isn't he?" Donna whispered.
"Yeah, but that's just how it works here."
"I bless each of you in attendance with fertile loins and pregnant bellies. Blessed are our children, for they are the future. We must never forget what happened yesterday even as we build tomorrow." He stood from the throne and slowly walked around the circle. He placed his thumb on each person's forehead in turn and murmured something.
When he got to her, Donna wasn't sure what to do. She smiled nervously at the elder.
"What a lovely pink couple," the elder said. "How many children do you have?"
"No, we- We aren't- We don't-" Donna stammered at the unexpected question.
"You don't have any children? Why, I bless you doubly my dear. May your womb soon overflow with children to please your husband."
"But-"
The Doctor put his hand on her lower back, so she didn't say anything else as the elder placed his thumb on her forehead and chanted unfamiliar words. She sighed when he moved on to the Doctor, relieved to be out of the spotlight.
"May you soon be blessed with a round wife and a large family to call your own." The elder placed his thumb on the Doctor's forehead and chanted, and then he moved on to the next person.
Donna whispered in the Doctor's ear. "Why did you stop me from telling him we're not a couple?"
"Because he would never understand us being the age we are, er, appear to be, and not married. Outsiders are rarely welcome here and the only ones who risk the long trek are those who are as desperate for children as the locals are."
Donna looked around at the crowd with renewed interest. Now that she was looking, she could see that the other outsiders were paired off in twos, and looked dirty, ragged and tired. They must have hiked miles just to get here.
"The blessing - does it really work?" she asked, feeling sad for the people all around her.
"They say it does."
She raised her eyebrows at him. "My womb is going to overflow with children?"
He shrugged, one hand going to the back of his neck to rub. "Well, I didn't say it was a sure thing. It's obviously not in our case. Now, are you ready for lunch? They serve this stuff called seetr at the feast. It's like pasta, and I think you'll love it. It's meant to be an aphrodisiac, but we're obviously safe from all of that nonsense as well."
"Sure," she said with a shrug, and reached for his hand.
Nine months later, Donna was rapidly approaching her due date with triplets. "I'm pretty sure my womb is overflowing now," she said, miserably, as she lied in bed next to the Doctor, one hand rubbing her enormous middle. "My bladder definitely is."
"Let me help you up, my lovely round wife," he joked, as he moved to do just that.
Turns out, both the blessing and the aphrodisiac were very real and very effective.