You got any headcannons for H50?
Danny only drinks the coffee Steve gets for him. It's nothing big really. He just doesn't trust Chin to get him anything that isn't some weird tea that he insists is better for him. And Kono, frankly, takes too much pleasure in getting everyone's coffee orders "wrong".
(Danny doesn't know that Steve switched out his original order ages ago, and that he's been drinking decaf the entire time.)
Steve always makes Danny do the paperwork. It's the least surprising thing about their relationship, to be honest.
Except for the part where Steve keeps checking in with Danny to "make sure" he's filling the forms in right. Up until such a point that Danny makes Steve sit in his office so they can fill out the paperwork together.
(Danny still does the majority of the paperwork, but Steve likes to think he's helping by providing moral support.)
Steve has a hardcore work out regiment that's frankly unnecessary so Danny makes it a habit to show up at Steve's in the morning in a bid to cut it short.
Sometimes Danny needs to bribe him with a case, or breakfast, or the lure of willingly letting Steve drive the Camaro.
He doesn't know that the simple act of him showing up at Steve's house is enough of an incentive.
Despite Steve dragging Danny short of kicking and screaming, to be part of Five-O, it's Danny that introduces them (on the job or not) as partners.
Steve's used to having a whole squad in his corner while Danny's had a revolving door of partners, and a tenuous relationship with HPD.
Danny's never had backup he could rely on purely because he trusted them to trust him.
(Steve takes pride in the fact that Danny's so openly calls them partners, and is grateful to deserve it.)
And also, because I'm a sucker for Danny's kids being Steve's kids:
When Grace gets her first visit from Aunt Flo, it's Steve that ends up with Grace.
Steve and Mary were close growing up, menstruation and all that it comes with is hardly new to him.
Whenever Grace gets cranky or is going through a million mood swings an hour, with Danny at his wits end (having sisters did not actually help Danny understand this any better), he calls Steve.
Steve talks Danny through what to do, and even shows up to mitigate some of Grace's more temptous moods, and mediates any disagreements had between Danny and his little girl.
And that's why, despite Danny's three sisters, Steve is who he calls for help.
Grace thinks asking Steve for boy advice is a good idea. Spoiler alert: it isn't.
Steve runs the kid's parents and his entire family through intelligence data bases, and when that comes up with nothing, Steve resorts to the old fashioned way, just to be sure. Of course, he actually has multiple guns and knows about a dozen ways to kill a man with his pinky, and the boy is suitably terrified.
Grace won't speak to him for a week after, and ignores her Danno for just as long when he can't hide his smirk fast enough.
Danny can't decide if he should yell at Steve or kiss him.
(Surprise! Danny gets into an argument with Steve about it as per usual and ends up on his back. Mutually, of course. Nothing revvs Danny's engine quite like Steve's protective streak over Danny's kids.)
Charlie wants his dad to come for Career Day, but knows his Danno is super busy saving people and doesn't want to bother him.
While Danny is convinced his kid is hiding being bullied, Steve knows the truth and wrangles Danny into going to Charlie's school during Career Day instead of in the afternoon (where Danny fully intends on giving Charlie's teachers a piece of his mind for letting his kid get picked on).
Charlie's too happy to have his Danno be there without having been told, and is now fully convinced that his Danno has superpowers that know when Charlie needs him.
Steve's too busy giving a smitten Danny heart eyes to take credit.
During a sleepover, Charlie insists that under his bed and closet is thoroughly checked out for any monsters. With Danny having been recently discharged from the hospital after a case, and still in pain, Steve takes up the mantle with aplomb.
When Danny finally wakes up from the nap Steve insisted he take while Steve handled the "monster situation", he ends up standing in the doorway of Charlie's room, staring at his son cuddled up with a sleeping Steve.
Charlie gives him the stink eye for even suggesting Steve be moved, and that's how Danny ends up squished on Charlie's other side as his son pets him and Steve both before declaring it was "Good night time".
In the morning, Danny's the furthest thing from embarrassed to have Charlie and Steve cuddled up on him. Hell, back ache or not, Danny makes them his famous chocolate chip pancakes and whistles the entire time.
Grace and Charlie take to calling Steve "Pop" after a soda incident when he took them out for ice-cream and cold drinks.
Danny remembers it vividly involving a can of soda that Charlie picked out, dropped and rolled down a small hill, before innocently handing it over to Steve to open for him. The result was as expected.
After the boy incident, Grace takes to calling Steve "Pop" whenever he pulls very Dad-like moves. From giving her crap about what she wears, perfecting his Disapproving Pop Stare, helping her with her homework, teaching her to drive and hugging her hello and goodbye. So basically, all the time.
When Charlie draws a family portrait for school, Charlie tells his class that he has his mom, his StepStan (lifted from Grace's vocab), his big sister, his Danno and his Pop.
Whenever either kid is mad, they call Steve by his name, and there's nothing that can get the kicked-puppy look out of him faster.