Focusing on shape:
* The umbrella was like a giant mushroom protecting her from the rain.
* The open umbrella was like a colorful parachute floating above her head.
Focusing on function:
* The umbrella was like a mobile roof, shielding him from the downpour.
* Her umbrella was like a personal shield against the elements.
Focusing on movement:
* The umbrella swayed in the wind like a dancing flower.
* The umbrella spun around like a spinning top as he walked.
Focusing on the feeling of protection:
* The umbrella was like a warm hug in the cold rain.
* The umbrella was like a safe haven from the storm.
The best simile depends on the context and the specific feeling you want to evoke.