* His eyes widened, astonished like a deer caught in headlights. (This is a classic simile, comparing astonishment to the startled reaction of a deer)
* She was so astonished, her jaw hung open like a dropped puppet. (This simile emphasizes the physical manifestation of astonishment)
* He stood there, astonished and speechless, like a fish out of water. (This simile compares the feeling of being overwhelmed to the awkwardness of a fish on land)
* The news left her astonished, her mind reeling like a spinning top. (This simile uses the image of a spinning top to convey the disorienting nature of astonishment)
* His face was as white as a sheet, his astonishment evident like a storm cloud on the horizon. (This simile emphasizes the visible signs of astonishment)
Ultimately, the best simile will depend on the specific context and what you want to emphasize about the character's astonishment.