Your quickest option is walking to MK and take the bus.
If you have time, we enjoy taking a boat whenever we can, but it would add a lot of time to your trip to go to DS first and take the boat.
Easiest is ordering an Uber or Lyft to pick you up right at the Contemporary entrance. Personally, I love using Minnie Van with Lyft while dining out in Disney. Walking back to MK and waiting for the bus back to your resort won’t cost you anything, depending on what shoes you’re wearing
Are we talking after fireworks? Because that’s gonna be NOPE. Can you do it? Yes. Should you do it? No. Don’t go have a lovely, relaxing, expensive dinner at CG and then walk into the post-fireworks MK exit mob to stand around and wait to be packed onto a bus like a sardine. Pay for the rideshare.