Broadway Market is a Victorian street market that's been around for ages. It's open every day of the year, with a mix of shops and stalls selling all sorts of goodies. On Saturdays, you can browse from 9 a.m. to 5 p.m., and on Sundays, it's open from 10 a.m. to 4 p.m.
It's right in the heart of the most trendy part of London, so it's also just a fun spot for people-watching.