I like what Martin is suggesting because it involves 2D and 3D, and would of course mean different bits and multi-stage carves.  I think maybe a guitar body is a bit advanced though, and certainly a larger piece that would require a larger machine.
How about something a little bit smaller that could fit on just about any machine?
One of my favorite Easel Gallery projects is the Death Star Trivet - Easel - Trivet - Death Star
I have done this project several times now, and I really fumbled my way through it and learned it the hard way.  A tutorial video on how to do it would be helpful to many, I think.  The way I do it is in several stages.  The first stage is all of the circles and ovals, and I do that at a depth of 3mm with a 1/8 bit.  The second stage is the line that goes across the middle, and I created a separate workpiece just for that, and use a 30 degree v-bit.  You CAN do both of those stages with a 30 degree bit, but I donât like the v on the circles and ovals, which is why I use a straight bit.
The third stage is do cut the âscoopâ, which is the concave circle, the 3D part.  I would like to see just how to create this part, because I have not done that.  The file that I have for it is not editable, I cannot choose a bit, depth per pass, or determine just how I want it done.  This is the part that I have struggled with, and boy did I make a lot of mistakes trying to make this work.
The fourth stage is to carve out the trench to about 10mm with a 1/8 bit.  Then I cut out the âincompleteâ parts where the rest of the Death Star havenât been built yet.  These are through cuts again with a 1/8 bit.  Then I do the sixth and final stage which is to cut out the whole outline to full depth.
Perhaps a simpler suggestion would be to do the Heart Dish Easel - Heart Dish
I have not done this project yet, so I canât offer any info on that one, but it does involve a 3d cut and multiple stages and bits.
I would really like to see a tutorial on how to go about designing and executing a 3d cut.
Looking forward to seeing what else is suggested.