Since you've disconnected the pump you will have open plumbing lines from the pool at the return and the skimmer. If the water level rises from snow or rain it will drain out of those. I cover the open ends with metal fly screen held on with zip ties to keep the critters out. I don't plug the return. At startup next spring I'll put the garden hose in the return opening and wash it out which will get rid of any vermin which might have got in. It has never happened in the past.

As to the skimmer, I put an old skimmer basket in it as a trap for debris or mice. Funny, your question about the skimmer cover plate. I had the very same question in another thread. My best guess for its purpose is to keep debris out of the skimmer.

If you do want to plug the return you could shop around for expanding rubber plugs, the kind that have a wing nut you turn to make the plug fatter.

One tip about the pump in the garage: if you have mice, they'll find it and build a condo inside it. I plug the open piping with a wad of steel wool.