Reference: http://www.alfawzan.af.org.sa/node/15847
Question: What are the deeds by which children can honour their parents after they have died?
Response: All righteous deeds which, if accepted by Allaah, will benefit the deceased; and the most important of them being supplication, [as mentioned in the hadeeth]:
…[in addition to] charity on their behalf, [as well as performing] hajj and ‘umrah – [so] all of these will benefit them. [Regarding] all righteous deeds, if [an intention is made to] gift their reward to the deceased, then they will [certainly] benefit them – with the Permission of Allaah.