Vomit works just fine, the acid in your stomach has a pH value of 1.
The downside is that vomit smells really bad and making yourself puke is not alot of fun.
You can also wash them with pure white vinegar and rinse it with bottled (or just really clean) water afterwards.
The problem with acid is that it will tarnsih your coins over time, if you plan to clean them only once it's not dangerous but if you plan to keep them and clean them again when they are dirty I advise you not to use strong acids.
To find the best treatment, you have to know which metal or alloy the coins are made of.