Instruments
Well-provenanced antique violins, cellos and bows have — over long periods, and net of dealer margins — historically held or grown value. Serious concert grand pianos hold up in a different way: they are used, treasured and passed on. Neither is a liquid investment; both are considered assets.
See Rare Instruments and Rare musical instruments as investments.
Art
Post-war and modern paintings with clear provenance and institutional exhibition history remain the deepest, most liquid market for private collectors. See Artful Interiors.
European decorative arts and furniture
Softer than a decade ago; but a considered buyer's chapter. Spanish antiques in particular are undervalued relative to their French and Italian counterparts — see A collector's guide to Spanish antiques.
Objects to consider — and to think twice about
Not everything marketed as an "investment piece" is one. See Luxury objects that hold value and What to buy instead of a Rolex.