What commonly illegal activities in most areas are legal in Amsterdam.

Alcohol

Legal, of course. The drinking age for beer and wine used to be 16, but as of 2015 it's 18 across the board. It's not legal to sell alcohol anywhere cannabis is sold, but some alcohol purveyors are 420 friendly, especially on the terrace.

P. S. Alcohol consumption outdoors is for terraces and parks only. Walking around with open containers on a non-festival day can land you with a €50 fine.