Tip: Go into a supermarket (Albert Heijn) or kiosk (AKO / Bruna) and buy a pre-paid public transport chipcard. It'll save you a lot of money on the trams and busses. They're the best option for getting around town, besides renting a bicycle (MacBike). Make sure you use the card to check in and check out every time you get on and off a bus / tram.
In the same street where you will find Nivo-Schweitzer, there's also a second hand shop for all kinds of gear.. Leica, Hasselblad, Rollei, etc. etc. But, the owner is kind of notorious for being weird, it's like he doesn't want any customers.
Anyways, if you go to the east, there's a very nice little camera shop called Foto Fransen:
, very nice people. They usually got a bunch of Leica's and nice medium and large format stuff. Sometimes you can get really lucky
(I picked up a mint noctilux there for a very nice price).
Food wise, stay away from the Leidseplein area, it's a tourist trap and things are overpriced and very mediocre. Zeedijk / Nieuwmarkt has some nice and cheap restaurants, Bird is nice if you like Thai. If you like Belgium beer, you should go to De Zotte, they have great food as well (you could go there just for the food).
Check out the Foam photography museum, they usually have a nice exhibit going on (plus they now have a popup shop/gallery near the Museumplein I think, which is free to enter).
Walking around the center is of course something you should do, I can recommend De 9 Straatjes area: http://www.de9straatjes.nl/
Lots of little shops and nice houses.
If you feel like going off the beaten path a little bit, the east has a nice area to walk around. Leave from the Central Station area and walk toward this place:
If you walk a bit further you can sample some nice beer in a brewery that is housed in a proper Dutch windmill (but check the opening times first). It's a short ride on Tram 10 back to Leidseplein from there: