The Altes Museum is the classical temple-like building on Museum Island facing the Lustgarten. Inside, the main draw is the antiquities collection, with Greek, Etruscan, and Roman objects arranged around one of Berlin's most elegant museum interiors.
Enter through the great rotunda, then work through the galleries rather than treating it as a quick add-on to the Pergamon or Neues Museum. The Hildesheim silver collection is tucked away deeper inside and is easy to miss if you rush.