If the foundry demands a minimum density then they probably
have provided in the PDK, their preferred density fill cell. This
would have dense features on all layers. The cells can be left
to float, or be tied to something benign (like substrate, where
it will sit anyway, and substrate-potential metal if you feel the
need).
I have preferred however to make my own plate capacitor
structures which use as many layers as possible, put over
MOScaps (ditto) and using them for on chip decoupling - at
least the area is not wasted. The yield and reliability guys
always whine about gate area, but you're forced to have
that area by the rules, so shaddap.