How long do you plan to use the site for?
If you have the luxury of time, by far the most environmental method IMO is to avoid anything chemical, any disposable materials (biodegradeable or not), etc.
The toilet we have at the cottage has a good quality permanent seat (ours is made from hard polystyrene as it's nicer to sit on when it's much below zero). The waste lands in a container and is sprinkled with a little sawdust. This is then is emptied periodically into an old chest freezer (painted green, may I add). It sits there for 1-2 years, by which time the bacteria/enzymes have broken it down into useable compost. This is then sprinkled on the vegetable patch.
Male visitors who consume gallons of beer are encouraged to leak in the forest, lest their urine make the other waste too damp and slow down its decomposition.
However in the next year or two I am considering a tree bog:
http://en.wikipedia.org/wiki/Tree_bog
And if anyone has built one, I would love to hear form them!