The first year was the easiest.

Alec knew Magnus had gotten him something, due to the smirk he had on his face when he got back to the studio flat that cold December 15th. He always had shown Alec what he had bought that day, even if it was just a pot of glitter or new suspenders (which Alec secretly liked, but he'd never admit that). That afternoon was different.
Alec had to find it.
He knew he shouldn't of course. Magnus would be terribly disappointed in him. But he couldn't help himself.
Once Magnus had gone out with a client that morning, Alec ran into the complex, using his extra key Magnus gave him. What followed was an hour of pillow squishing, drawer opening, and closet swimming.
He found it, coincidentally, in the corner of the drawer where he kept his spare set of clothes.
(He means in case he ever spills something on his clothes...not like he'd stay the night...)
Alec, despite the fact his curiosity was at an all time high, did not open the unwrapped box.
For exactly 8.47 seconds.
He slowly opened the box and found a light blue onesie, with a long curly tail and cat ears. Well, Magnus certainly did have a sense of humor.
When he found out, (Alec isn't good at keeping secrets) he wasn't mad. Surprisingly, all he said was,
"Just you wait, Alexander Gideon Lightwood. Just you wait" and stalked off into the kitchen for the mugs of hot chocolate he had in the microwave.

The second year was not as easy.
Magnus knew, now, Alec's tendencies. And he was determined for it to be a surprise this year.
Alec, however, was even more determined.
Magnus had gone so far as to put PROTECTION WARDS along the perimeter of his flat, but Alec happened to know a certain short red-head who could easily break them, as she had been practicing her new power. What followed was yet another hour of pillow squishing, drawer opening, and closet swimming.
Eventually, he found it under a heavily imprinted glamour under the Magnus's side of the bed. (Not like Alec had a side or anything... pshhh no)
This time... Batman knee socks. And Rainbow Dash knee socks.
Again, Magnus has a sense of humor.
That Christmas when Alec opened the box, Magnus already knew. (Clary wasn't good at keeping secrets either).
"Well hell, Alec. next year I'm just not going to get you anything at all.

But, that wasn't necessarily true.
Alec knew that just one more year of finding his gift would break his dear Magnus, and then he wouldn't even try to hide it anymore. He knew, and he wanted it.
But when he arrived outside his studio that frigid morning, he found nothing. No protection wards, no lock spells, not even a mouse trap (which me knew Alec was terrified of). The only thing left was the, "Keep off the grass" sign by the neighbors door.
He took no caution.
Magnus had called him earlier that same morning, saying he would be in Idris to help his, "Unrefined government" with something and to not call him during that time. It seemed fishy, but what could Alec say; it gave him time to look for the gift.
He stepped inside the entry way (small, but Magnus-Ified) and set his black (what other color) jacket on the table, shaking his head to get remaining bits of snow off. He stalked into the kitchen to begin his search.
What followed was yet another hour of pillow squishing, drawer opening, and closet swimming.
But, unlike the other years, he didn't find it.
Eventually, after checking the bedrooms, bathrooms, living room, kitchen and laundry room, Alec was done. Maybe Magnus had gotten the best of him this year. He collapsed on the couch.
"OWWW!" he screeched, hitting his head on something rock hard on the pillow. he sat up and saw a small bump in the pillows on the couch.
He got excited.
He slowly unzipped the side of the pillow case, taking his hand and pushing it into the side and grabbed the small box. He took his hand out and brought it up to his face.
It was a small, red velvet box.
Just as he put his hand on the top of the box, ready to open it, a voice interrupted him.
"Oh Darling, that was supposed to be a surprise."
Alec jumped onto the couch, whipping his head around as he saw Magnus materialize behind him, faking a disappointed look, smiling in his eyes.
"Wha... you weren't here..." Alec stuttered catching his breath.
"Its called a glamour, sweetie. Now, you were snooping. Isn't that a federal crime?" he smirked.
"Not if I have a key." Alec said, matching Magnus's smirk.
Magnus shook his head. "Just give me the box and let me do this properly. But don't expect any of the gooey stuff." he said, holding out his hand.
"What?"
"I said give me the box."
Alec tentatively handed it over. Magnus took it and grabbed his hand, pulling him off the couch into a standing position.
"I said to not expect the gooey stuff. All I'm going to say is, I love you, and I want to spend the rest of my long, long, LONG life with you." Magnus's smirk got wider, casually turning into a grin. Alec's face was just slack.
"Alexander, will you marry me?"
Alec's eyes stare into Magnus's, and he sees no hesitation there. Only love.
So Alec doesn't either.
He nods slowly, taking the ring from the box and putting it his finger.
"Well I didn't find the gift this year, did I." he smiles.
Magnus kisses him.