Some of the special things that Adrien has noticed about Marinette include:
-- The way she forgave him after she thought he had stuck gum on her chair on the first day of school, and he apologized and explained that he was just trying to *remove* the gum that Chloe had put there.
-- He appreciated the scarf he gave him for his birthday, even though due to Nathalie's interference he doesn't know it came from her and he thinks it came from his father. I'm sure he would be doubly impressed if he knew that she said nothing to change what he thought only because she could see how happy he was that his father had given him something nice and personal, instead of the pen set he had gotten for previous birthdays (which shows little thought put into the gift by his father). She was SO sweet to do that!
-- Her winning the fashion competition held by his father to design a bowler hat, and her cleverness in signing the work by including her monogrammed initial in the design.
-- He *really* appreciated the poem she wrote for him in the Valentine's Day card she sent him, even though he still doesn't know who wrote it -- but that's NOT HIS FAULT.
-- Her graphic design skills in creating Jagged Stone's new album cover. He is so impressed that he asks her to autograph her work!
-- Her skills at videogames, easily beating him after he had easily beaten Max, who everyone thought was the ultimate videogamer.
I could probably list a few more if I re-watched some of the episodes again. I know there must be other examples.