There are at least one or two similes from this poem that are quite easily understood. They have to do with the second and third ages of man in Shakespeare's poem.
The first has to do with the schoolboy "creeping like snail." The simile means that the boy is going very slowly. He is going very slowly to school because he would rather be somewhere else. In this age of life, the boy's life centers around having to do things he does not want to do.
The second has to do with the lover "sighing like funace." This is meant to show how a young man at that age is as hot with passion as a furnace is with actual fire.