It’s canon that Niki is not good at romance or portraying himself like a "normal" husband. But there are things Marlene notices that he does to show that he cares:
He always makes her favorite foods. Even when he fails miserably or he dislikes said food. Ex: mushrooms. Marlene is a mushroom girly and Niki hates them.
Niki always drives and always opens the car door for her. Not that he doesn't trust her to drive, he believes that his woman shouldn't have to worry about driving. Especially when she's married to a Formula 1 driver.
Niki can get protective and vice versa for the famous couple, especially after Niki's accident. It's the little subtle gestures he makes in public, being a celebrity can be very dangerous. Ex: hand on the back when escorting her everywhere, whenever they walk alongside the track Niki puts himself between her and the road/cars, being protective at the after parties or the press conferences. He never takes his eyes off of her.
Slowly Niki has gotten better at remembering their anniversary date. The first time he forgot, a week later he came to her workplace holding flowers and wearing the brown suede blazer she loved. The second time, he took her out to a fancy restaurant and told her to order whatever she wanted. The last time he didn't give her expensive gifts, take her out for dinner, or try making up for it. The last time it was Marlene who forgot and Niki was the one who remembered.
Niki tries every chance he gets to make her laugh. One time Niki made her laugh while they were eating. Usually, Niki is the one who's taken away with work so these dinners together were very rare. When he made a joke about a certain British Formula 1 playboy, Marlene had laughed so hard that the wine she was drinking came out of her nose.
Bonus Marlene tidbit: Marlene's love language is acts of service/gift giving. Platonicly and romantically. She loves taking care of Niki as well as her friends and family. For James' birthday, she had ordered a special (very expensive) whiskey and a new drink cart for his collection of assorted fine wines and bourbons. It was difficult for Niki to adapt to someone taking care of him. He grew up independent, most of his previous relationships never lasted long for various reasons for him to get close enough. Marlene snuck her way in and broke down his walls. For that reason he had learned to accept what Marlene wants, she gets.
