A sitting duck is something or someone that is easy to criticise or target.
This is a way of wishing someone a good night's sleep.
Someone who is as sly as a fox is cunning and experienced and can get what they want, often in an underhand way.
This idiom is used to describe someone the speaker does not believe has the ability or resources to handle a task or job.
If someone is small fry, they are unimportant. The term is often used when the police arrest the less important criminals, but are unable to catch the leaders and masterminds.
If you smell a rat, you know instinctively that something is wrong or that someone is lying to you.
Someone who is a snake in the grass betrays you even though you have trusted them.
Advice or medicine which is of no use.
A person who promotes something that doesn't work, is selling snake oil.
A snipe hunt is a situation in which someone is tricked into chasing after something that does not exist. The origin comes from a hazing ritual in which someone would be led into the field to catch snipe (an imaginary animal) in a sack while the other hunters pushed the snipe in that direction. The other hunters would in reality leave the hunter there and go on their way.