Royal Hospital
A retirement home for soldiers created by King Charles II. Tours around the listed building and grounds are regular and include the museum (which can be visited separately) whose exhibits contain military memorabilia donated by Chelsea Pensioners over the years.

A retirement home for soldiers created by King Charles II. Tours around the listed building and grounds are regular and include the museum (which can be visited separately) whose exhibits contain military memorabilia donated by Chelsea Pensioners over the years.
Check the official site before travelling; hours and prices can change.
More museums

Sir John Soane's Museum
Free, tiny, deranged — Soane's own house preserved exactly as he left it in 1837.

The Wallace Collection
Free national museum in a Manchester Square townhouse — armour, Boucher, the Laughing Cavalier.

The British Museum
Free, vast, civilisationally heavy — go for one room, not all of them.
Source: wikivoyage