Rollercoasters wouldn't be easy, as you'd have to learn lots of calc and physics, to know materials and safety test rides virtually.
Toy designing would be ok, probably more marketing issues than anything else. You'd probably have to learn about circuitry, maybe a little AI.
I've done architectural drafting. Moreover, I evaded it. EXTREMELY boring and annoying unless you're at the top of the chain, like Ghery. Mechanical I enjoyed, because I could use the program for other things, like this:
[youtube]http://www.youtube.com/watch?v=ev0u4qa6QWw[/youtube]
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When I allow it to be
There's no control over me
I have my fears
But they do not have me