Darry: He gets you a necklace in the shape of a tree, because he thinks it symbolizes growth and strength, which is what your relationship has consisted of.
Steve: He gets you guy tickets to a rodeo, because he knew you liked excitement, and you got to spend time together and go on a date!
Two-bit: he finds a set of matching underwear... And they're meant for a couple... The guys has Mickey Mouse on the butt and the girls has Minnie Mouse on the butt so he thought it would be something you'd get a kick out of.
Johnny: he gets you a Jean jacket just like his. One so you guys can be the cute matching couple. And second because you never wear a jacket and it makes him mad because he wants you to be warm because he loves and cares about you a lot.
Ponyboy: he makes a scrapbook, filled with a bunch of different things inside, like the day you first met, and more memories and pictures and a letter at the end saying all the reasons he loves you with a rose that he had bought just for you.
Sodapop: He buys something that you'd never expect. He buys a German Shepard puppy because he says he wants something to protect you when he's not around. He already talked it over with your parents and they were ok with you having a dog. You guys think of names and eventually name her Willow. (Picture above because I think this one needs a visual lol)
Dally: he gives you an old switchblade of his with his initials carved into it, he said that he wanted you to always be able to protect yourself, and he liked the idea of you being badass carrying around a blade.