Simple STL manipulation software?

All I want to do is put a hole or four in one of these: http://ift.tt/2dNA4J9 my kids can put them on legos. Bizarrely, the ProE/CREO that I have at work can’t do s**t with stl files or convert them to solids. I know that Blender could do it, but that software has a pretty step learning curve.Any suggestions? TinkerCAD maybe? http://ift.tt/2dlKXPf