These are some useful acts for your deceased parents:
1. Fulfilling all their debts whether religious debts of worships which they missed (Prayers, Fast, Khums, Hajj etc) or financial or moral debts to people.
2. Paying Sadaqa or charity on their behalf. Charities with continuous benefits (Sadaqa Jaria) is much better like water will, Masjid, Husainia, orphanage, school, etc).
3. Arranging Majaalis on their behalf.
4. Reciting Quran or paying to people who can recite on their behalf.
5. Reciting Ziyarat Ashura and other Ziyarat on their behalf.