Gidday Marvin,
I enjoyed your gallery. You are doing very well.
When it comes to post processing, I do not think that there are any hard and fast rules. Do what looks good to you. If you only want to play with contrast and saturation that is fine if you are satisfied with the results. Images from the D70 can sometimes benefit from a little work with tone curves - I use Curve Surgery Pro (NEF) or Curve Therapy (jpeg) for this. Here is a link to an article on sharpening. I like this article because it keeps it simple.
http://www.bythom.com/sharpening.htm
If you are going to crop the lake shot I would take a little of the right and top of the image.
With the possum shot, I would be trying to pull both possums in close by cropping.Jordan suggested softening the background to get the possums to stand out against the background, this would probably work quite well. One of the easiest ways to do this is make these adjustments, is to use layers and the various eraser tools. Simply duplicate the layer and erase the parts of the layer that you do not want to apply the changes to. The magic eraser is very useful, by utilising the threshold value in the information bar you can get control over the tone range that is erased. Explore the various effects of brightness, contrast, saturation and filters (lens blur is a good one) on this layer until you find something that you like. The ability to adjust the opacity of the layer in the layers palette is a useful and powerful tool if the intensity of the adjustments is a little too strong for your tastes. A few small contrast and/or brightness adjustments to the possums in the original layer can make them really pop. When satisfied, crop and sharpen. I gave it a quick crop and did not mind the shot at all. Cropping for 10x8 or 5x4 might be the go.
This one of many methods that can be used to acheive the same thing. I think that the trick is to keep your changes small.
I have made one assumption here. That is that you use a flavour of Photoshop.
Cheers
Matt