Elena and Cas
Castiel had started out watching the festivities from the rooftops, out of the reach of the crowds. Over the millennia of his existence, a lot of human behavior became boring after a time in their meaninglessness and repetition. But he had found he liked watching large group celebrations like this one. He liked the cacophony and the high emotions and the sense of community everyone seemed to feel - there was something spiritual about it, in a way.
When he got bored of just watching, he flew down and just wandered from one scene to the next, fascinated by the array of color and costume and movement and noise. He watched a thrill ride cycle through over and over for at least an hour, trying to figure out why humans would line up to spend a few minutes on a precarious contraption that just flooded their bodies with adrenaline and nausea. Curious creatures, humans. At another booth, he watched a young woman painting on the faces of children and adults alike, and when he walked away it was with a rainbow-feathered wing that faded into an equally colorful scaled pattern adorning one cheek.
He was watching a man frying up churros when he sensed someone familiar not far away. He flew closer, landing just behind her and reaching out to catch her hand before he could lose her in the crowd. "Elena!"