A voodoo altar would be an important centerpiece. It sounds like you're on the right track, and items that Grapegrl suggested would be central to an altar. Other things that would be used in voodoo ceremonies are bottles of liquor (or liquor bottles with colored water

), tobacco products (cigs, cigars), money (coin or paper), and sweets (candy!!!!). These things are all common offerings to the different voodoo loa (spirits).
Burn some incense (the earthier the better) in a safe container on the altar so the space is smelly and a bit hazy. If you can draw or otherwise mark symbols on the floor in front of the altar, that would be cool too. If you can't find saint statues, saint candles would be great (tall glass jars with different colored wax and a saint's image printed/pasted/painted on them). Different colors of candles are important also, even if you don't burn them.
You might try searching for voodoo altar pics on the web, but I don't know if you'll find any. It's bad mojo to photograph altars, or so I've heard. I'm going to New Orleans for a week on Saturday (yea!), so I'll have a look-see at the voodoo shops and the voodoo museum to see if there's anything outstanding that hasn't already been covered in this thread.