Congrats on the Dane. Welcome to three full years of hilarity AND aggravation. What is funny now, will become EXTREMELY obnoxious in a few months.
I know as I have an almost 9 month old Dane myself. You will inevitably become familiar with two terms:
1: Zoomie. This is when they barrel around the house, bouncing off of everything (and everyone) in their path for no apparent reason. The bigger they get, the more damaging this becomes. On the tail comment, keep your flat screen T.V. higher than the dog's tail. Someone in DOL, had their screen destroyed by their Dane's tail.
2: DRI. This stands for "Dane Related Injury". This is most apt to occur when #1 is in full tilt. These include scrapes, bruises, scratches, and yes can include broken bones. Rufus bolted by me while I was re-laying out a rug on the floor, cracking me hard in the nose just this morning. No damage to me - THIS TIME. I was vacuuming, and he hates the vacuum.
Now is a good time to get what is called the "Horking pig", and "Obnoxious chicken". These are two noisy toys that oink or make a squawking noise when they are being squeezed. These become good methods of knowing what your pup is up to. Eventually he will outgrow these toys, which will be marked by their quick destruction. When he gets bigger, I recommend going to Tractor Supply or other such place that sells horse toys, and get him a Jolly Ball. Rufus eventually got to the point where anything I bought was scattered all over the house in a matter of minutes. The Jolly Ball has survived his jaws so far. Even when they no longer hold air, they keep and spring back to their shape. The handle may or may not be chewed off depending on how determined your Dane is to dismember stuff. Rufus figured out that he can carry it around by the handle, so it has survived - for now.
DO NOT FEED YOUR DANE PUPPY FOOD. They are extremely susceptible to joint problems, and puppy food has too much protein. I don't know what you're feeding him, but I've been recommended to feed mine, and do feed him a brand called 4health from tractor supply, after I received an article regarding Purina Beneful dog food killing over 200 dogs with no recall to date. I feed my dogs the salmon and potato brand in particular to stay away from grain.
These guys are extremely susceptible to bloat as adults. It is recommended to get him used to the idea of keeping quiet for about an hour before and after eating. A good crate is recommended to keep them quiet.