1. Black Current/Black Food Coloring - this will make your cocktail dark and creepy. Activated charcoal, also known as activated carbon is used primarily to treat poisoning and can interfere with medications. And ladies on birth control pills, this has been known to nullify the effectiveness of your method and you don't want to be in for a big scare a few months down the road. So play it safe and stick with the black food coloring. (see UPDATE below for more info)
2. Squid Ink - used mostly as a coloring agent, this is best used in moderation to avoid an overly fishy taste in your cocktail. Gives your drink an eerie, slimely feel. EEEK!
3. Dry Ice - probably the easiest and most readily available. Throw some around your bar area at the last minute to make it appear that your cocktail is smoking. Never handle dry ice with direct skin contact as it can burn your skin due to the intense cold temperature. While it will not affect the taste of your cocktail, you don't want to accidentally consume it. Keep it in a bowl close to your cocktails to get a cool effect.
4. Grenadine/Cherry Liqueurs - give your drinks a bloody effect with grenadine or any cherry liqueur. Plus side is also the great flavor that these bring. Red tones vary from bright red to deep, dark red depending on the brand used.
5. Edible Glitter - make your drink sparkle above the rest. Easily available at most grocery/party supply stores or you can make your own. A simple touch of orange, black or silver glitter will turn that ordinary cocktail EXTRA-ordinary!
Please read directions on these products before using to ensure a safe experience. Oh, and I must warn you ... use of some of these ingredients may have a effect on you later. Nothing serious, but don't freak if you have a scary bathroom experience.
UPDATE: To read more about safe ingredients in cocktails, visit https://www.cocktailsafe.org