Here's a partial list of references which people mentioned in the comments:
I noticed Hyun Woo, while window shopping in Germany, pick up a flower pot similar to the Edelweiss from CLOY which Jung Hyuk gifted to Se Re. (I'll update this with the episode and timestamp, later.)
Hyun Woo's Suzy smile… (??)
Someone mentioned Vincenzo was using the pen gifted to him by Cha Young
And another comment mentions Hae In's mom was watching CLOY in ep 4,
@Tom mentioned that there was also a My Liberation Notes nod on the subway in ep 8.
@10years noticed Beom Ja's line, "that guy looks like Leslie Cheung" and that the actor played the role of Leslie Cheung’s ghost in a small indie Korean movie.
@OfftoDramaLand caught the way HI waved bye to BHW outside their countryside front gate was similar in CLOY to how Captain Ri awkwardly waved back at Yoon Seri in North Korea.