Offerings to the Ancestors
The best offerings to the ancestors are ones designed for them personally. The traditional offering is food or drink that they would have liked, but if you want to go further, donate time or money or resources to a cause that they would have approved of. This is easier for relatives who died during your lifetime, and who you knew personally - just think of some organization they would have approved of, and give to that group in the name of your relative. When it's people who are many generations dead, or perhaps so far back as to be nameless, it's more difficult. Start with their ethnicity - is there an organization for people of that ethnicity who need help, perhaps in countries other than their original homeland? Barring that, is there a harsh issue that is troubling the descendants of your ancestral people who still live in their own country? Could you donate to an organization who is trying to help? Imagine them looking down at all their modern descendants - who would they be most worried about? That's a good place to start.
If you're looking for some generic organizations, try these:
Cultural Survival: Helping to protect indigenous cultures and traditions.
The Emeril Lagasse Foundation (since food is such a tremendously good way to connect to the dead).
American Refugee Committee (if your ancestors migrated to a new and difficult place).
The Fuensanta Arismendi Plaza Scholarship Fund (which my mother started).
Save The Frogs (don't ask, it's at the behest of one of my ancestors)