shops for groceries- I think Cas would be pretty enthusiastic about buying all kinds of cool foods, but Dean would be really insistent that he buy the food cause Cas always comes home with weird stuff
kills the spiders- Cas refuses to kill spiders, and gets really upset when Dean does it. Dean is all aggravated because Cas doesn't even take the spiders outside, he just wants to let the spiders stay with them
comes home drunk at 3am- Is this even a question? Dean, obviously.
makes breakfast- Dean, because he loves to cook. Cas will occasionally surprise him though.
remembers to feed the fish- Cas. They're his fish, after all.
decorates the apartment- both of them. They like making the place theirs.
initiates duets- It depends. Cas can usually play an old record and Dean'll start singing on his own. But if Dean catches on that they're singing a duet, he'll stop, so Cas has to be subtle.
falls asleep first- Dean. He's well trained to get as much sleep as he can whenever he can, so he pass out quick (this is of course assuming he's not in one of his mega guilt kicks because then it's hard for him to sleep)
shops for groceries- They shop together. Sam is there to make sure Dean buys actual food. Dean is there to make sure Sam doesn't get nothing but health food.
kills the spiders- They pretty much just kill them as they see them. Dean is a bit more squish happy about it though.
comes home drunk at 3am- Usually they come home drunk at 3 am together (possibly after a quickie in the bar bathroom, depending on how drunk Sam is), but on occasion they might be out separately.
makes breakfast- Dean, pretty much for the same reason as above. And he'll never admit it but he loves how pleased and surprised Sam always is at Dean's cooking abilities.
remembers to feed the fish- They don't have fish.
decorates the apartment- Dean. Sam'll put up some pictures, but it's Dean who makes it feel like a home.
initiates duets- They'll belt out to Dean's classic rock collection, but they don't really sing duets.
falls asleep first- Usually Sam, because Dean sleeps best when he can hear Sam snoring beside him, cause then Dean knows Sam's safe.