The terme or baths are amazingly well preserved, you can actually see the hypocaust under the mosaic floors where sections have fallen through.
Some of the most extensive and well-preserved frescos are in the Sede degli Augustali and the beautiful Casa dei Cervi. You can see stunning mosaic floors in several buildings (most notably the women's baths) and the wall mosaic in the Casa di Nettuno e Anfitrite is particularly impressive.
I went in October and it wasn't too crowded, though even then there were a few large tour groups going around which were best avoided if you actually wanted to get a good look at things.
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