Excellent question and one which deserves all the detail possible, as our Western culture tends to take away, instead of give credit, to the wealth of information that we can acquire from our grandparents.
The field of Social Sciences is divided into twelve (sometimes more) sub-fields. Each of them is strongly connected to history, which is one of the sub-fields. To have grandparents that have lived through ANY historical period other than our own is extremely important: it gives you a first-hand witness account of what really took place during that given day and time, being that your grandparents are none other than PRIMARY sources. This means that they are living history; they are the first witnesses of what went on- not a single textbook, expert, nor professor would ever be able to reproduce what your grandparents can tell you in one conversation.
From the sub-fields of economics and education- both under Social Sciences- you can get first hand information from your grandparents in a variety of hot historical topics such as (depending on the age of your grandparents)
- effects of the economy on jobs
- effects of education in job acquisition
- changes in the educational styles
- how money (or the lack thereof) affected educational programs
The sub-fields of Law and Political Science can help you elicit questions to your grandparents that will paint a picture of how society dynamics took place back then: what was allowed and not allowed? who were the "bad guys" and "good guys" of that time? What movements surfaced? What prompted those movements to change? How does society compare then and now?
Think about the sub-field of Psychology- ask them who were the "crazies" that made the news. Maybe they witnessed the likes of Bundy, or the Jonestown Massacre. You could establish correlations based on events that took place back then: do specific time periods show more psychotic activity than others? Were there specific illegal drugs that were popularly used then? How were learning disabilities labeled during their time in school? What did they see happening to students with disabilities in a regular school day?
Social work and Communication are other two sub-fields from where you can get information. In social work, particularly, it would be interesting to draw information about family dynamics, child rearing, and even how child abuse was treated by the social support systems. Communication is a superbly fun field of its own, dealing with technology, and with the state of the depending on the age of your grandparents).
Other sub-fields to consider are
Take the time and really brainstorm about the wealth of information that is available from a first-hand source. You will not be disappointed, and you have way more credibility as a researcher obtaining first accounts from witnesses than quoting a generic textbook.