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.
Hannekes Boom is one of the nicest waterfront café in Amsterdam to have a beer when it's sunny. Close to Central Station, the place isn't crowded by tourists, but you need to be there early to get a seat.