I would put the lid down on the toilet and get a trash can with a lid and foot pedal. Then just add a few toys and a baby, and shut the door. If, at some point she starts lifting the lid to the toilet and flushing her toys, you will have to decide if you want to put a lock on the toilet seat (maybe just for shower time) or just teach her not to mess with it. I used to put random things in the floor like a shoe box and a hair brush that he wasn't used to playing with, and that would keep his attention long enough for me to do what I needed to do. I also sometimes took a bath rather than a shower, because he liked being able to see me and talk to me. If your little one doesn't mind the water, you might even put a towel or bath mat down in the shower (so it's soft and not slippery) and let her splash while you shower. That is, as long as she isn't likely to fall over from sitting. If so, maybe try that in a few months when she's a little stronger.
Kat, wife to and mommy to (Dec 07).