Pepper has Tony's original arc reactor mounted in a glass case engraved "Proof that Tony Stark has a heart" and gives it to him.
At the lakeside funeral, Pepper sets that same engraved reactor on a wreath of flowers and lets it float out onto the water.
The first object that kept Tony alive becomes his memorial. The engraving that started as Pepper's joke in 2008 reads very differently eleven years later.
Source: Den of Geek: Avengers: Endgame - Who Was in That Important Spoiler Scene