I’m a minimalist when it comes to jewellery. I don’t even wear my wedding ring these days. However, I do sport diamond studs (on sleepers) that I wear 24/7. If I ever switch them out, it is for single pearls, or gold hoops.
Gold necklaces (chains of varying widths and lengths) and pearls - single, double, triple strand - are classic, but can be styled in non-classic ways. I think of certain wrist watches and bracelets (diamond tennis bracelet, gold circlets, charm bracelets) as classic.
Signet rings, gold bands, diamond solitaire, eternity ring - classics.
Can you think of anything else?