One of the highlights of all D&D games I've ever played was
Finally levelling to the point I could cast 7th level spells, while the party was trapped on some awful demiplane where strange creatures were coming out of cracks in reality and just beating us to shreds.
Learning Mordenkanen's Magnificent Mansion.
Casting it so the party had some where to go rest and escape.
Taking the party on a tour of the mansion and getting super prissy about it. "Here's the kitchen. Here's the living room, no feet on the couch! The guest rooms all have private bathrooms, everybody better wash up, and send your dirty clothes to be laundered before you touch ANY furniture!"
That game continued to epic level and that character (who I had played starting from first level over the course of YEARS) created this spell using the 3.5 Epic Spell development rules. It is the best spell ever.
I think, the first casting of MMM always needs a dedicated scene where the rest of the party get to explore what the wizard has made for them. It's a massive opportunity for creativity with the players, and as a DM you can just take a load off and let the wizard player run the session for a while, explaining all the cool things they incorporated.
The first time I cast MMM in 5e, it was with my pyrophobic gnome librarian wizard, Neff. She build a special room for each member of the party to cater to their needs, and I built a map for it (we were on VTT)
That's awesome. I love the attention to detail for each party member (which is exactly what we would expect from you). I bet there's a story behind the dragon skull and the piano. I wish I still had my mansion maps from that epic game (it was about ten years ago).
The piano is actually there because we needed a ballroom to entertain some NPCs for a political summit. Several of the servants in the mansion were skilled in instrument use, so we made a small area for performers.
The Dragon Skull is in the sand of a fighting/training pit for the party's fighter. She basically took out a red dragon single-handedly during one of our adventures, and it's a replica of that dragon's skull that we couldn't take away as a trophy because the dungeon collapsed.