Assuming it is someone I actually give a shit about (parents/gf/son) I always get some shitty cheap card unrelated/inappropriate to the occasion (I got my dad a card for a 6 year old last birthday with Jake the pirate on it and added a 6 to it

) but just put in a private message that is personal/humorous depending on the occasion and whether I was drunk when I wrote it.
If I had to give a Christmas card to someone who was into training I'd throw in some gym references/jokes; if it was from me, as someone who trains, to someone not into training, I would shelve my hobby and speak to them as a person. Jesus I waffled there.