The best thing you can do for the parents after death is pray to Allah for their forgiveness and elevation of their status in the Heaven. Another thing that will benefit the parents is your good deeds. Allah says in verse 15 of Surah Al-Ahqaf, “Now (among the best of deeds which) We have enjoined upon man is goodness towards his parents. His mother carried him with pain and gave birth to him in pain, and his gestation and weaning (period) is thirty months. And so when he attains to full maturity and reaches (the age of) forty years, he (that is righteous) prays (to Allah), saying, “O my Lord! Enable me to be grateful for your blessings with which you have graced me and my parents”.
Do you know you pray for your parents five times daily? Actually, while offering prayer, you also pray for the forgiveness of your parents. When you say “Rabba-nagh-fir lii wa ley-waa-ley-day-ya walil-Mu’mi-niina Yawma yaquu-mul-Hisaab!”, you are praying for the forgiveness of your parents. It is verse 41 of Surah Ibrahim and is translated as “Grant Your forgiveness unto me, my parents and all the believers on the Day on which the (last) reckoning will come to pass (or the final account will be established)”.
Dua for parents after death will not only serve as the source of their forgiveness but also it will benefit you in the long run. Firstly, praying for parents is a virtuous deed and you will be rewarded for that. Secondly, if you pray for your father and mother, your children will also pray for you after your death. Thirdly, praying for your parents means fulfilment of a religious obligation. So, you will get a sense of satisfaction.
Dua for parents after death is also given in Arabic here.