The Doctor's TARDIS

Indestructible, spaceflight
Crew Time Lord 2 loyalty (Tap any number of
Doctor, Master, Melody, or Rassilon Planeswalkers
you control with total loyalty 2 or more: This Vehicle becomes an artifact creature until end of turn.)
Whenever The Doctor's TARDIS is crewed
during your non-extra turn, you may pay {6} any
number of times. For each time, take an extra
turn after this one.