Curate, connect, and discover
draw something with them
write something with them
make an edit or other graphics of them. screenshot edits where you add your headcanons or other changes to their design are great too even if you don't commit to the new design
revisit your favorite scenes or even reconsume their entire source
go hunt for pictures of them you didn't save yet
go look for new fanart, fanfics, or other fan content made by others
gush to other people about them
talk about them in general even if it's not a gush. share your favorite fun facts, talk about their source, or share some headcanons
find someone to roleplay your f/o for you
make a journal page dedicated to them
write them a letter (and maybe write a response letter from their perspective too)
listen to songs that remind you of them. you could also make a 2010s style AMV of them with that song