Dear Paige
I would fill low planks with layers of plywood cut into strips. They are then glued (construction urethane adhesive) and then stapled (remember the narrow crown air driven staples ?) to the low spot in the floor. Then take a belt sander or floor edger with really coarse sandpaper and sand the edges of these plywood shims so they are smooth to the rest of the floor. Then you can install the 1/4" plywood floor over top and paint and stain as you wish.
Don't try the quick fix of those floor leveling compounds, in just a few months they will be crunching under your feet as they crack to pieces on the wooden floor boards. These are used to level concrete floor and they have a checkered past of failures anyway.
Oh, and you could always just install 1/2" plywood underlay (might have a tough time finding a nice grade-try BC douglas fir). This stuff is stiff enough to span a few low boards without any trouble. This is what we use when we are installing a new hardwood floor on a really wonky softwood subfloor. Funny the thicker plywood is not that much more expensive than the 1/4" underlay I mentioned before. Be sure to give the edges a bevel, before you install in so that the edges are smooth and don't meet at different levels. You can even staple down the 1/2" plywood. But in this case screws would be better in this thicker material. And just leave the screws flush but exposed for the neat but industrial look.
As always your Most humble servant, Joseph, the Wood Floor Doctor.
