I only have one daughter (23 months) who is not very wild and plays with her toys well alone. So I just tell her that mommy is going to take her shower now, and then do it. As soon as I'm out of the shower, I open the bathroom door so she can come in if she wants, and she knows this.
For a time she used to watch as either my husband or I showered, but she would get wet from the spray, or would try to bring toys to hand to us. For a while I would also use the baby gate to keep her on the upper level with me, and she would play with our dog up there while I showered. She also went thru' a period of wanting to sit on the counter to watch me put on makeup; she's less into this now, but it was fun for her to handle mommy's makeup items, look at herself in mirror, etc.
Probably the biggest piece is that I would go about my business (cleaning, on phone, etc) while she played from the earliest age, going from room to room and therefore leaving her alone in a room for brief periods of time. So she can handle herself alone in a room for a brief period of time and not freak out, scream for mommy, etc.
Also, I know that when you have 2 or 3 kids, this might not be an option any longer! Good luck!