Our son recently saved up his allowance to buy a small remote-control helicopter, so today we made some rings for him to try to knock over and pick up with the copter.
STL file: http://www.geekhaus.com/makerhome/day75_helirings.stl
Tinkercad link: https://tinkercad.com/things/iO6Lqo7BN18-day-75-helicopter-challenge-rings
Thingiverse link: http://www.thingiverse.com/thing:181495
Settings: MakerWare low/.3mm, I think it was under 30 minutes for all three together.
Stuff you might want to change: The medium-sized one is so light that it knocks over from the wind of the helicopter. Next time a larger base would be nice.