Smack dab in Covent Garden is the London Transport Museum. It’s filled with things that go vroom for little (and grown-up) transport fans. Very good gift shop with a nice selection of posters. The museum is near the Royal Opera House. And while there aren’t any loverly singing guttersnipes out there nowadays, there are usually street performers (fire jugglers, musicians) and a few places to sit and have a snack.
Getting there: Northern line to Leicester Square or Piccadilly line to Covent Garden