A theatre hidden in the man made forest on the outskirts of Amsterdam. An impressive setting with impressive shows. Watching an interesting piece under the stars is quite a memorable experience. Further more it's a nice bike ride leading you past rowing waters, fields to play picnic in or a pancake house if you feel like something warm and sweet.
The city's biggest flea market is hosted the first weekend of each month in and around a gigantic warehouse in the industrial area of NDSM in Amsterdam Noord. I love to snuff around the hundreds of stalls to find unusual props. Taking the boat to cross the IJ and leaving the old centre behind is very refreshing. Make sure to also have a drink in Noorderlicht while the sun sets.