I did not do a lot during my years in undergrad and I had to explain why in my essays.
I did a lot of campus activities, campus organization leadership positions, etc. I did research with a professor and had good LORs. Even with a double degree I had to really sell the fact that I had a good understanding of what social work would require of me and how my credentials set me apart from the other applicants.
During my year off from school, I am actually getting into volunteering now that I have the time.