First, find out whether you have to. Some foundries want to
put down the fills themselves using in-house mask prep folks.
Then, if you are on the hook for it, look for cells in the layout
library like "DENSITY" or "FILL". They will probably be small,
maybe 2x2 contact, via1, ..., viaN with criss-crossed metal
strips and maybe some poly, N+, P+, active features for
those.
I like to make little multilayer poly/m1/m2/... capacitors and
use those to push up fill% while also getting some supply
decoupling inside the bondwires. Might as well make some
use of the "white space".