What are some similes and metaphors in the novel Crispin: The Cross of Lead by Avi?

The author uses similes and metaphors throughout the text. Figures of speech that employ comparisons between unlike things, as both of these do, may help make the visual aspect of a narrative more dynamic by conjuring up images. They may also relate some element of the speaker’s everyday existence to larger issues or abstract concepts. A simile uses “like” or “as” in making the comparison, while a metaphor is a direct comparison.
After the bailiff burns down his hut, Crispin climbs up a hill at the edge of town to get a better view and decide where to go. Looking down, he uses two similes, one comparing the landscape to a tapestry and the other comparing the sky to garments.

Before me—like some rolled-out tapestry—was my entire world beneath a sky as blue as Our Lady's blessed robes.

In describing the dull routine of their lives, Crispin uses the metaphor of the millstone for time. A similar figure, referring to the mills of God or the gods, derives from an ancient Greek saying that refers to divine retribution.

Thus our lives never changed, but went round the rolling years beneath the starry vault of distant Haven. Time was the great millstone, which ground us to dust like kernelled wheat.

After a narrow escape from his pursuers, as he is accused of theft, Crispin sleeps outside and wakes up in the morning to a thick fog. He uses both a simile and metaphors to compare it to wool, a toad, and rotten hay. Another metaphor calls the sun a jewel.

I woke to a wool-like world of misty grey. Thick and clammy air embraced me like the fingers of some loathsome toad Sounds were stifled Solid shapes were soft as rotten hay. No sun jewelled the sky.
https://archive.org/stream/CRISPIN_457/crispin_djvu.txt

Similes and metaphors are literary devices that use comparisons to help with descriptions in writing. Similes are phrases that use the words "like" or "as" to compare two things, while metaphors are more direct and do not use "like" or "as."
There are plenty of similes used in Crispin: The Cross of Lead, such as the following example, quoted from Chapter 8:

The things the priest had said made my heart feel like a city under siege.

This line comes after Father Quinel tells Crispin that he has been declared a "wolf's head," meaning that anyone may kill him without consequence. Crispin's heart is like a city under siege, because he is overwhelmed and in great danger.
In Chapter 8, Father Quinel also tells Crispin that his name is Crispin—before this point, the boy had only been referred to as Asta's son. This leads to this line of narration in Chapter 9:

It was rather like a new garment that replaces an old, desired but not yet comfortable.

Crispin uses this simile to describe how he feels about his new name; he definitely likes it, but he's not quite used to it yet.
Now let's move on to metaphors. The following metaphor comes from Chapter 2:

Night was a mask for outlaws, hungry wolves, the Devil and his minions.

Here, without using like or as, Crispin compares the night to a mask, because the darkness keeps things hidden.
Those are just a few examples, there are plenty more to find!