Common:
* As brittle as glass: This is the classic and most common simile for something fragile.
* As brittle as dry leaves: This emphasizes the fragility and potential for shattering.
* As brittle as old bones: This emphasizes the fragility and age of the object.
Less common, but effective:
* As brittle as a dried-up twig: This emphasizes the brittleness and lack of flexibility.
* As brittle as a snowflake in the sun: This emphasizes the fragility and potential for quick disappearance.
* As brittle as a promise whispered on the wind: This emphasizes the fragility and impermanence of the object.
More specific:
* As brittle as a dried-up cookie: For something that is easily crumbled.
* As brittle as a frozen lake: For something that may crack under pressure.
* As brittle as a dried-up flower: For something that has lost its life and strength.
The best simile to use will depend on the specific context and what you want to emphasize.