Give nose/forehead kisses: Nami, especially when she gets used with him suddenly stretching his head to sneak on her.
Become jealous more often: None of them, they just trust each other too much. Nami certainly notices it more, when others are getting swayed by Luffy’s personality, but she says nothing. She knows he only has eyes for her.
Pick the other up from the bar when they’re too drunk to drive: Nami, because odds are she’ll never get that drunk, and while Luffy isn’t a huge drinker he can very well mistake some drinks for fruit juice. Luffy probably wasn’t the one to call for her, but instead the bar owner did, because he’d been trying to “help around” and “serve the food” since he had no money to pay for his consumption. Spoiler alert: he wasn’t helping. Good thing Nami sneaks her contact number on multiple pieces of paper in his wallet, pockets, bento, etc.
Take care of on sick days: Both do, as shown in canon; Luffy being super worried and determined to find a doctor for Nami, trying to cheer her up… and while it’s not usual for Luffy to get ill, he can get seriously injured, poisoned or sick (and sometimes due to his own stupidity), to which Nami responds by holding his hand and trying not to panic, and getting slightly mad at him if it was his fault.
Drag the other person out into the water on beach day: Probably Luffy, even though he himself can’t just jump on the water without some life-protective measures. Nami loves swimming and is happy to go along most of the time, but her skin is sensitive so she can’t do that repeatedly. If she prefers to stay by the parasol, Luffy will make time to sit beside her and play in the sand.
Give unprompted massages: Neither do it very often, but a few times Nami has had the experience/curiosity to give Luffy a “massage”. She is both weirded-out and amazed by how his rubber skin stretches and bends with ease, so it ends up being a hand-massage on herself.
Bring the other lunch at work: Nami, because there are days when Luffy is just so excited to explore a new island that he forgets his bento, and Nami is the one to hand it to him. Also if they need to make food, Nami wouldn’t let Luffy even approach the kitchen after the disaster of the Zou/WCI trip.
Have a better parental relationship: Both, Luffy would play with them all day and Nami would be such a loving mother. She would teach them her geographical and financial skills, and they’d grow up to be carefree, confident and smart, mischievous kids. No one would even think to mess with them. They’d be super proud and loving towards their parents.
Try to start role-playing in bed: I think Nami, although not always would Luffy understand what exactly she was trying to do. Nami can be really kinky and experimental in that regard.
Be an embarrassingly drunk dancer: Neither, because while the embarrassing one is Luffy, he doesn’t need to be drunk to do that. While he’s not much of a drinker, Nami can hold her liquor so she wouldn’t turn embarrassing, and she’s got quite the moves.
Firmly believe in couples costumes: Most probably Luffy. Nami is kinda picky with the costumes, and Luffy, sulking, doesn’t understand why Nami wouldn’t want to dress like, let’s say, the Power Rangers or a Transformer. But he’s cool with not having couple costumes if they don’t come to an agreement (needless to say, Nami is too).
Break the expensive gift rule during Christmas: Pfff, none. Luffy would give her a handmade gift, put together with the help of Usopp and Franky, and Nami knows Luffy is a simple guy who likes simple things. She’d give him something from the heart, but preferably cheap.
Make the other eat breakfast: Neither. Luffy doesn’t even need to be hungry to eat, and Nami loves breakfast food, especially fresh fruit. The only situation I see where one of them may insist is if Nami’s sick or not feeling well, Luffy snatches something off the kitchen and brings her. If she doesn’t want it anyway, Luffy will eat it instead.
Remember anniversaries: Nami, totally. If let be, Luffy will forget his own birthday. He doesn’t care for “special dates”, he’d bring her gifts out of the blue, in any “non-special” day. Nami is the one to plan their anniversary commemorations and Luffy “remembers” her birthday because Usopp and Robin make sure he’ll give her something on that day.
Bring up having kids: Neither, they have a silent agreement that when it happens, it happens. Both would like kids but they’re not in a hurry.