Lily went home after fifth year determined to act like a Muggle.
She folded her robes (carefully and neatly, of course) and put them away, pulling out her old t-shirts and jeans from dresser drawers that hadn't been opened in five years. She stashed her wand under a pile of books, where she knew Petunia wouldn't think to look. Tuney hated to read.
Tuney appeared in Lily's doorway as she was attempting to shove her trunk in a corner. In true Tuney fashion, she didn't offer to help, instead watching quietly. That's why it took Lily by surprise when Tuney asked softly and a little hopefully: "Lily? Does this mean you don't want to be a witch anymore?"
Lily startled and it took every ounce of effort she had to fight the instinct to whirl around and pull her wand out of her back pocket (where, she reminded herself, it wasn't anymore). Instead, she settled for slowly straightening up.
"No…" she said slowly, wondering how to explain to her little sister something she didn't even completely understand herself. "It's just that I want to get away from magic for a little while. Spend the summer with you and Mum and Dad, you know? We all won't be around forever. And there's nothing wrong with being a Muggle!" she added fiercely. Petunia frowned as she tried to process the apparent non-sequitur.
"What about that boy?" Petunia never called Severus by his name. Lily's face darkened.
"Severus," she said delicately, taking care to call him by his full name, rather than the nickname of "Sev" she had bestowed upon him. "Does not matter." Petunia opened her mouth to ask a question, but thought better of it when she saw the look on Lily's face.
"Look," Lily said, kneeling down in front of her sister. "Let's have a fun summer, just you and me, okay? Like we used to. We can play in the yard and catch toads and climb trees. We can even build a treehouse."
Ordinarily, Petunia would have dismissed the possibility of a treehouse as childish nonsense, and it was true that she felt that both of them were far too old for such things, but the prospect of spending a summer with her sister, without magic getting in the way, was too appealing for her to turn down.
"You'll spend the whole summer with me?" she challenged. "No magic talk?"
"No magic talk. We'll spend the whole summer together." Lily promised, crossing her heart.
And so they did. James' letters to Lily went unopened, and by the end of the summer Lily's bedroom window was covered in "presents" that angry owls had left after she refused to let them in. Severus knocked on the Evans' door twice. The first time, Lily slammed the shades down on the window and left him standing on the porch in the pouring rain when she saw who it was. The second time, he thought better of himself and ran off before anyone had a chance to get to the door.
Meanwhile, the most spectacular treehouse the town of Cokeworth had ever seen was taking shape in one of the backyard trees.
Sirius went home after fifth year determined to act like a Muggle.
It wasn't too hard. He had basically been acting like a Muggle at home his entire life, purely to defy his parents. This was just upping the ante.
However, he realized he had a distinct lack of Muggle clothes, and not the faintest idea how Muggles usually dressed. So his first order of business was going to the Muggle shops to Muggle-ize himself (he resolved to write to James and tell him about the new verb he had invented).
After getting spectacularly lost in the Tube (twice), and failing spectacularly to impress a couple of Muggle girls who were sitting near him on the train, he was starting to rethink his plan when he finally made it to a row of Muggle shops that seemed to have all he needed.
Unfortunately, family outsider or not, he was still a Black, and Blacks did not ask for help. After wandering aimlessly around the shopping center for three hours, all he had to show for himself were three magazines with Muggle girls in revealing clothes posing stiffly on the covers, two pairs of trousers that may or may not have actually fit him, and one jacket that he only bought because the salesgirl had said it would make him look "even more gorgeous". He had invited her out for a ride on his motorbike, but upon hearing that he didn't actually have said motorbike with him, she had called him a tease and stomped away.
By then, he was almost out of Muggle money. He usually changed some of his money to pounds at the kiosk by Platform 9 ¾, if only so his savings wouldn't mysteriously "go missing" every time his mother wanted a new portrait done. However, after his shopping expedition, he was down to his last ten pounds, and he didn't think the Muggle staff would appreciate it much if he tried to pay with Galleons.
So back into the Tube he went, clutching his shopping bags, and as soon he got home, he set to work. Systematically, he ripped the pictures out of the Muggle magazines and stuck them to the wall all around the big Gryffindor banner he had put up over Christmas his first year. As he was finishing up, he heard a deep, bullfrog-like voice behind him.
"Oh, Mistress will be so angry, so very, very angry, what will she say, what a shame, what a disgrace her first-born is…"
"Get out of here, Kreacher." Sirius ordered, throwing one of the now-empty shopping bags in the house elf's general direction. Kreacher sank into a deep bow and muttered something about having to obey before leaving.
Satisfied, Sirius leaned back and surveyed his handiwork. His eyes fell on the corner of a photo, half buried under an avalanche of schoolbooks that likely wouldn't be touched before it was time to return to school. Cautiously, he pulled out the photo. He grinned as he saw his three best friends surrounding him, all laughing as James attempted to smooth down his hair for the photo. With one last Permanent Sticking Charm, he affixed the photo to the wall in a place of honor, before throwing on his new Muggle clothes and heading down to dinner.
Three weeks later, James nearly fell over laughing at the sight of his best friend standing on his doorstep, in woefully mismatched and ill-fitting Muggle clothes, rain streaming down his face.
"Hiya, Prongs." Sirius grinned. "Mind if I come in?"
And that was when the Grand Muggle Experiment (as Sirius dramatically called it) officially came to an end.
They met on Platform 9 ¾ at the end of the summer. Lily looked like she wouldn't mind never speaking to Sirius or James again, but Sirius approached her, if only to stop James moaning about how she never answered any of his letters.
"Hey Evans," he said softly. "How was your summer?" She shrugged indifferently.
"Alright." She answered. "Acted like a Muggle. Built a treehouse with my sister. You?" She pointedly ignored James.
"Alright." He echoed. "Acted like a Muggle. Got kicked out. Moved in with James." She nodded and just for a minute, Sirius thought he saw a glimmer of understanding in her eyes.
"Well." She said finally. "See you around, then." As she turned to haul her trunk onto the train, Sirius had the queerest feeling that perhaps he had more in common with Lily Evans than he had ever thought.