During summertime Denmark is all about flea markets. One of them is located in Frederiksberg, right behind the city hall.
The market is every Saturday from 8 am to 15 pm from April to October. You can find a bit of everything here and since most of the sellers are ordinary people who have emptied their closets it's really easy to find great and useful stuff at reasonable prices.
Flea markets are also great places for shopping toys and second hand clothes for children.
You find the market in Smallegade 1. (The square behind the city hall).