One thing that is always in my toilet bag is this rubber multi size basin plug.
After many years of travels its now comes as no surprise to me to either find the basin , or bath plug, is missing.
That makes it a problem to shave, wash your face, or launder your clothes ,unless you have got your own plug handy.
Use With Integrated Mixer and Basin Outlet
I have even found it useful when I have been faced with an integrated mixer and basin outlet with a leaking seal.
As you will see from the photograph below many of those fancy chrome units just lift out so you can use the rubber plug.
Additional Travel Hack
If you can’t lift out the basin outlet duct tape will provide a waterproof seal.