For the best casual lunches I always go to Van Harte or Greenwoods, they have the best sandwiches and a really cosy Amsterdam atmosphere. The canal district in general is good to stroll around when you'd like to grab a bite. The best pizzas I've had are at the Italiaan in the Western part of the city, and when you go out there make sure you try the truffle pizza - simply the best. For a slightly more alternative venue you should hop on a ferry to the Northern part of Amsterdam, across the canal. Pop into PLLEK or Hotel de Goudfazant and you will have an amazing time for sure!