Victoria and Albert Museum - especially the Medieval Gallery and the Raphael cartoons.
Have lunch somewhere nearby. Maybe Ceru in Bute Street or Comptoir Libanais at the end of Exhibition Road, close to the tube.
Walk to Harrods and check out the Food Hall. Or walk across Hyde Park if it brightens up. Great exhibition at the Serpentine Gallery (Cecily Brown - I think it’s still on?)
Take the bus to Piccadilly and check out the National Gallery. Alternatively go to Tate Britain.
Or the Courtauld at Somerset House, next to Waterloo Bridge. Have a coffee there too.
Or across the river to to Tate Modern or the Imperial War Museum.
Or there’s the Uber boats down to Tower Bridge or upriver to Battersea Power Station - now a shopping mall.
Or you could take a bus to Westminster Abbey - sometimes they have organ recitals late afternoon . Or check out St. Paul’s.
Soooooo many options….. so much to see and do - but so little time!