The weekly Troedelmarkt at Strasse des 17. Juni is pretty well known in Berlin and it's also mentioned in practically all travel guides. What these guides never mention is another market that is also on Strasse des 17. Juni but on the other side of the Charlottenburger Tor - the Künstlermarkt. Every Saturday and Sunday (starting from May, it seems) local artists are selling their handmade work here, which ranges from paintings, leather bags, wood work and jewelry to toys, clothes and iphone sleeves. My favorite artist is a woman who makes table ware out of old silver cutlery - of which, sadly I wasn't allowed to take pictures. But I will probably buy something from her in the coming weeks, so I will be able to show you anyway.
The most popular stand (especially on sunny days) is the Wunderseifenblasen stand of Peter & Pat. They make giant soap bubble blowers from rope and bamboo sticks. Very simple and extremely effective - not to mention funny and addictive. All you need is a large amount of soapy water and a little wind and there it goes! My 5-year old son was so fascinated by them that I had to buy him one (a small one - orange, of course).
He happily went off to try it out, and what do you know? Even he can do it!