Australia has compulsory voting in the sense that you have to show up to a polling place and have your name crossed off the roll.

What you write on the ballot paper is up to you (oh yeah, voting is done with pencil and paper, not voting machines), and there's no penalty for casting an invalid vote, and no way of checking whether or not your vote is valid - you just put your paper in the box and walk out.

To properly donkey vote in Australia you usually draw sufficiently large genitalia on the ballot to ensure no one could fill it in after the fact.

I suppose it could also not be genitalia. Like that's theoretically an option.