For someone who's a bit spacey

Estar en las nubes

es-TAR en lahs NOO-behs — To have your head in the clouds

"Perdona, ¿qué decías? Estaba en las nubes."
Sorry, what were you saying? I was daydreaming.

Literally "to be in the clouds" — used for someone distracted or lost in thought, not necessarily in a negative way.

"Basically my permanent state during long, boring bureaucratic meetings, if I'm honest."— Graciela