I don't know if someone has already said this, but you could have this huge fountain in the front yard and in the back you could build a stage will all of the instruments on it.
Yes, a stage is indeed cool. But because my sims don't really use it as a stage (if there are fans around they become 'fans on stage' quickly) I decided to make a seperate instrumental room. I've planned a main heart of the mansion where all basic necesarly things are put together: living room, dining room and kitchen and within reach on the second floor are the bedrooms. The rest of the rooms are ment to be skill rooms, where all skills are put in seperate rooms with every possibility to learn a skill (logic books as well as a chess table) will be put together. That way depending on the character of the sim they can learn the skill in their own decired way.
The living room will mostly be mixed together out of hidden skill objects. And the gym will also have a television, pool and stereo in it. I agreed that learning athletic can be best done at the 'real' gym in town, because the training will be improved, but for those athletic sims who already have reached level 10 with those weirdo wishes to train 12 hours in a row it doesn't matter if they do it in the gym or in the home gym. Saved them traveling time.