Great topic. When the mastiffs are young we use a variety of things to keep them from chewing the furniture. We don't use marrow bones but instead knuckle bones. Keeps them busy for hours!
Also exercise--a tired puppy is a happy puppy and happier owner

We don't run or walk mastiffs when they are young--too damaging to the joints possibly but we do swim them! When Sparkle was a puppy she would swim about 30-60 minutes a day! She LOVES her pool. She would sleep for hours after it. It is the best for young dogs as it is non weight bearing but great exercise for growing joints.
We feed a raw diet so our dogs have bones every day to eat. I know what you mean about clean teeth. Vets always mistake the ages of our dogs based on their teeth. I have never paid for a teeth cleaning yet

Other great busy toys--fill a kong with peanut butter or that soft cheese.
We also use crates so when we are not around, puppies cannot get hurt or destroy things.