When we moved the plumbing around in the bathroom, we ended up leaving sizeable holes in the floorboards from the previous pipework. These holes weren't going to be hidden by bathroom furniture, so they stuck out like an ugly missing piece.
I'd had previous experience filling holes in our last house and found this tried and tested method to be the best by far! It's also super quick and super easy too!
Makeshift a slice of wood that fits snugly into the hole. I'm using off-cuts of old floorboard here, but any decent cut of wood should be fine! Make sure to hammer it down a little so it's super wedged and secure so it won't budge.
Using a multi-tool (or a chisel) cut the wood so that it's level with the rest of the floorboard.
Using a coloured wood filler (preferably one that's a similar colour to the existing floorboard) fill any gaps that are around the plugged wood. I find it easiest to just cover the whole thing in filler.
Sand and admire! Your work here is done.
What method do you use for filling holes in the floor?