I second the magnatile suggestion. Way more popular then duplos here.
We just have some mixed duplo bins I got at yard sales, etc but the most important pieces we have are the base plates. I have found our duplos got much more play when we added those.
Also agree with pp idea of adding accessories for the unit blocks. Or try intoducing the block into her regular play- like help her build a zoo for her little animals or bridge for her cars, etc.
"Someday, I am going to go to bed without toys on the floor and sippys in my sink. Things will stay where I put them and I can have nice, fragile items on display. But when I do, I will look around with a wistfullness in my heart and miss the days where vibrant imaginations ran wild and there was evidence of life happening around me all the time!"