Today, I got two mails form Mathematica. The first one contained another activation key. Remember that I had troubles on the activation dialog. The second one was an advice to download all 1.1 GB of Maple 9 once more, uninstall and re-install. There seems to be problem on some Windows systems causing a blue screen of death.  I did not experience this problem. Most likely, the problem affects updates only.

I played with some more 3D graphics. There is indeed an option for level lines, if we use an implicit contour plot.

The command was

ContourPlot3D[z - x^2 + y^3, {x, -1, 1}, {y, -1, 1}, {z, -1, 2},
  Contours -> {0}, Mesh -> {5}]

By the way, I cannot export this to PNG, since my „version does not support this“. I hope this only a restriction of trial version. What you see are screen dumps I got with the snipping tool. Mathematica does very nice and advanced contour plots. Here is another example

With Maple, the same plot looks like this.

Mathematica has a clear advantage here. It produces smoother, better looking 3d plots with less effort and runtime. The command for Maple was

with(plots); 
implicitplot3d(f = 0, x = -2 .. 2, y = -2 .. 2, z = -2 .. 2, 
  axes = boxed, style = patchcontour, grid = [40, 40, 40])

Euler cannot do this. You can only get a wire frame model. On the other hand, Euler can show this plot in anaglyph mode in 3d.