Pitru Paksh :
1. Do Annadaan (donate meals)
2. Donate milk in Dharamsthan.
3. Feed Cows at Gaushala which have been abandoned.
Feeding a healthy cow nearby your house is also good.
But feeding cows at Gaushala or any abandoned cow is more advisable.
The above can/should be done throughout the year but if possible should be done at least once during Pitru Paksh.