As i was answering this I noticed that the prompt list had TWO 19s on there so you get a bonus question for freeeeeee
19. An ideal free day for them would look like? What are they doing on that day?
Okay, assuming they have nothing else going on that day, it actually starts the night before when either Shun sleeps over at Dan's house or Dan sleeps over at Shun's house. If Dan's place, it's a lazy morning b/c Dan's mom is making pancakes - if Shun's place, Dan gets up first to make breakfast to bribe Shun out of bed with it.
Then, depending on the weather, they're going out to do things. I think they usually trade off who gets to pick the Activity(TM) whenever they hang out - a compromise they made during childhood and never stopped honoring. So if it's Shun's turn, they'll head to the park, the woods, the beach, or anywhere similarly outdoorsy and in nature. If it's Dan's turn, they're hitting up the amusement park, the arcade, they're hopping from cafe to cafe, the movie theater. They spend most of their day outside if they can help it - but if the weather is bad, it's video games and movie marathons. If they're going out, they pack a lunch unless they happen to be going somewhere with food and when they're ready to go home, it's sleepover time again : )
BONUS 19. What was the moment in canon that made you go "Yeah, I ship it"? If they haven't met in canon, what influenced you to ship the characters?
I mean, it's gotta be the flashback in episode 12, Shun and Dan's battle, the way Dan immediately pivoted into helping Shun get back to his mom. . .you can tell they had history. You can tell Dan really cared about Shun (at least in the Japanese version. . .English Dan why did they butcher your writing like that) even when he was frustrated with him. I think it's significant that in the Japanese dub, Dan was the one that pointed out that the battling dimension didn't stop time, it only slowed it, and by trying to stall out forever Shun was hurting his mom more. He knew. He fucking knew Shun just wanted to save his mom. But he couldn't or wouldn't let him ignore reality. . .THAT'S LOVE BABY!!!
21. What would be some pet names they would give each other?
I don't think they use pet names frequently, if at all. I think if they're calling each other pet names it's in teasing/jest, not serious at all. Otherwise. . .there has to be a moment. THE Moment(TM). Dan says 'babe' in a fic I wrote but Shun was too out of it to be embarrassed by it
22. A headcanon you have for this ship?
Dan and Shun are actually very comfortable doing parallel play and most of their non-Activity (TM) hangouts consist of Dan playing videogames or fucking around on his guitar and Shun reading a book or doing some sketching and they can co-exist like that for hours while rarely/barely speaking and be totally content. Nobody would look at Dan and think he could just vibe like that but as long as his hands and/or brain is busy, he's good.