Someone is on a plane/transit for an entire daylight period of Sukkot and accidentally only brought along their etrog, leaving left the rest at home ;~
Is there anything to do with just the etrog?
ד' מינים הללו מעכבין זה את זה שאם חסר לו אחד מהם לא יברך על השאר, אבל נוטלן לזכר בעלמא (בין ביום א' בין בשאר ימים)
These four species restrict each other such that if he was missing one of them he does not bless on the rest, but he takes them as merely a remembrance. (Shulchan Aruch OC 651:12)
So you should just pick up the Etrog during the day as a remembrance to the Mitzva.
Aruch haShulchan there (:28) quotes two opinions regarding if you should shake the Etrog or just pick it up.