As with all my theories, this assumes jongkey is real and have been together for years.
I have feeling that the birthday gifts they give each other are not things you buy in a shop. Sure they probably still give gifts, but the real presents are more sentimental actions to make each other happy.
For Key’s birthday, Jonghyun was styled up differently than his usual clothing and he was all smiles taking tons of selfies. I think Jonghyun’s gift to Key was this action. Key is into fashion and putting outfits together, so what better gift to Key than to be his model for his birthday, everything on Jonghyun’s body being hand-picked by Key as an expression of his creativity and love. A secret gift that only they share.
So for Jonghyun’s birthday, I think Key’s gift was the date to Taeyeon’s concert. I imagine it would be very hard for them to always be hiding, to never be able to go out in public and have a normal date.
Jonghyun is such a emotional being, I think hiding all the time wears
him down, it makes him sad and Key said Jonghyun really needed this
present. On this day they got to arrive together in the same car in broad daylight, waving to the fans. They got to sit next to each enjoying the concert together without having to have the other members there as a cover. They took pics together and shared them on instagram without having to disguise that they were there together. When you can’t have the things other couples easily enjoy, can you imagine what an amazing gift this would be?