One of the most beautiful gifts you can give someone who has died is sincere dua, and it isn't limited to the funeral prayer or the days right after — you can make it any time their memory comes to mind, for the rest of your life. The Prophet ﷺ taught a dua at the funeral prayer that captures what we hope for someone: 'Allahumma-ghfir lahu warhamhu wa afihi wa'fu anhu, wa akrim nuzulahu, wa wassi madkhalahu' — 'O Allah, forgive him, have mercy on him, grant him strength and pardon, be generous to him in his resting place, and make his entry spacious.' You can adapt the pronoun for a woman or for multiple people, and you don't need Arabic fluency to make it meaningful — a simple, heartfelt 'O Allah, forgive them, have mercy on them, and grant them Paradise' carries the same weight. Many people also like to recite Surah al-Fatihah or Surah Yasin and gift the reward to the deceased, or simply pause in quiet moments to ask Allah's mercy on someone they loved.
Q&A · Death & Afterlife
What du'a can I make for someone who has passed away?
References
Sahih Muslim 963
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.