- green drink (lime mineral water, green KoolAid, lime cordial...)
- lime or lemon sorbet/sherbet/gelati
- dry ice (optional)
Take the most revoltingly green drink you can find (I used lime mineral water, the original recipe called for green KoolAid), put it in a punch bowl, and scoop green (lime) sherbert/sorbet/gelati into it. When this starts to melt, it gives a convincing scum on the top of the drink. Lemon sorbet works also. Adding some dry ice gives you some swamp fog as well. Just be careful that no-one touches the dry ice!