Plugging large hole in bathroom floor
Get a can of expandable foam from your hardware store. If they have it get the one for extra large gaps. Also pick up a tub of ready mixed tile grout. 
If the hole is really deep the foam may fall through before it had time to expand so you may need to stuff some newspapers into the hole to give the foam some thing to rest on until it had time to set up. Get as thick a layer of foam in there as possible for insulation. 

After it has set, trim it off half an inch below floor level and trowel in the grout mixture level with the floor.