I'm going to start by printing a 15mm cube, because that's something that my current printer (Makibox) can't do reliably.

Then I'm going to print what I refer to as the "pin grid" object, which is a grid of 5x5mm pillars topped by a plate. I designed this specifically to be really, really hard for FDM printers, and so far it's failed on the Makibox (repeatedly) and even on a fancy Stratasys uPrint (although that was partly user error).

Then I'll try my buckyball-inside-a-buckyball-inside-a-buckyball-inside-a-buckyball-inside-a-buckyball-inside-a-buckyball-inside-a-buckyball object. Even the fancy Stratasys printer can't handle this because the STL file is too complex (around 400MB) and Stratasys aren't interested in fixing their software.

Finally, I'll get on to some useful stuff. Top two here are landing gear doors for a Durafly Vampire and potentially little gears to use with that (depends how precise the Peachy can be).