If you're as snug as a bug in a rug, you are feeling very comfortable indeed.
If you use a sprat to catch a mackerel, you make a small expenditure or take a small risk in the hope of a much greater gain.
Someone who's a spring chicken is very young, often inexperienced.
A stalking horse is a strategy or something used to conceal your intentions. It is often used where someone put themselves forwards as a candidate to divide opponents or to hide the real candidate.
Someone who will not listen to other people's advice and won't change their way of doing things is as stubborn as a mule.
Someone who's exceedingly strong physically is said to be as strong as an ox.
The straw that broke the camel's back is the problem that made you lose your temper or the problem that finally brought about the collapse of something.
If someone is sweating like a pig, they are perspiring (sweating) a lot.
A person's swansong is their final achievement or public appearance.
If someone is swimming with the fishes, they are dead, especially if they have been murdered.
'Sleep with the fishes' is an alternative form.