There are a few chemical methods that you can use to brass-coat copper coins. They look very impressive at open houses for schools, so I've had first-hand experience here. (*Don't do this at home before looking up this method in at least two other places, and make sure you know what all the ingredients are!)
1. Dissolve zinc sponge in boiling sodium hydroxide. This is something you won't want to do on your stovetop.
2. Put your copper coins in the mixture after removing it from the heat. The shinier they are, the shinier the brass will be.
3. The zinc will react with the copper and coat the copper coin with a silvery layer of zinc. If you just want a silver penny, stop here.
4. Put a zinc-coated penny in a Bunsen burner flame. Zinc has a low melting point (it can melt on your stovetop), so it will liquefy on the coin without dripping off. Certain liquid metals can dissolve certain other metals, so the copper from beneath the zinc will mix together and the coin will turn yellow in about ten seconds.
5. Drop it in a glass of water to cool and you're done!
Most "gold/silver" finds (especially circulation) for copper coins are going to have been messed with chemically this way, and they're not worth anything more than novelty value.