There are some really interesting shapes within manmade vehicles. they will test your ability to draw with good perspective and proportion and the final result can be very satisfying.
You don’t have to draw every intricate detail if you don’t want to - if time is short, just focus on getting the overall shape and shell correct, and maybe a few key features.
An old tractor from a low vantage point (simplify the engine block and focus on the shadows of that big tyre tread)
A wooden dinghy boat in a nice landscape (focus on the boat and just add a very sketchy line drawing for the backdrop)
A traditional red London bus (worried about perspective and straight lines - go deliberately wobbly with a pen)
the front of a classic Mercedes car
or go for a modern fighter jet instead (great angle on this photo showing the detail of the undercarriage and landing gear)
this subway train coming towards you (looks difficult and time consuming but you could use a charcoal or carbon pencil to add lots of the darks quickly - so the train is emerging out of very little detail around it)