Another good possibility is running instances with a fast killing class like rogue, fury warrior or hunter.

A level 60 character for all twinks up to level 40 and a level 70 character for all instances past that. The reason for the higher level in later dungeons simply is the killspeed or exp/hour you want to achieve.

Always start the instances early:
Deadmines 15-20
Stockades 20-25
Gnomeregan 25-30
Scarlet M. 30-40

I suppose you could continue with Zul'Farrak and Sunken Temple up to 50 and then Dire Maul. I would favor Dire Maul over Strat or Scholomance because of the easier pulls imo. Though I have not advanced that far yet.