Firstly make sure they are worth next to nothing! Cleaning up a valuable coin (eg a 1933 penny) will seriously reduce its value. Very dilute hydrochloric acid is excellent at cleaning up copper coins. For silver or cupro-nickel coins use dilute nitric acid. Rinse them thoroughly in water afterwards or else they will quickly tarnish.
coke... ive seen ordinary coke dissolve chewing gum before now. and when (santa) used to leave new coins at the bottom of my stocking, he used to soak them in coke to make them look really new.