We can play at the artificial grass pitch next to the Amsterdam Sloterdijk station. There are a couple of crucial points to keep in mind about outdoor games though.
1) It is not possible to reserve outdoor pitches (I've tried everywhere I could think of, but couldn't find any pitches that could be booked in advance. If you know of one, please let me know). This means that the pitch may be being used by other people when we want to play, and so there is no guarantee that we can actually play there when we want to play, and we won't know before we actually get to the pitch.
The benefit of the Sloterdijk pitch is that it is not in a residential area which may increase the chance that it is available. Last year we were able to play every time we went there (at times with some other people joining us) but we may not be so lucky this year.
So the thing is, there is no guarantee and there is nothing that can be done about it (so don't blame the organizer if the pitch isn't available).
2) It may rain. A bit of rain isn't so bad but more than a bit becomes problematic, also because fewer people will show up then and we may not have enough players for a good game. So if it is expected to rain when we play the game will be cancelled.
3) The goals on the Sloterdijk pitch are bizarrely wide so that it becomes relatively ridiculously easy to score. I'd advise to narrow the goals by placing bags or coats that can be used as posts.
4) The upside of it being impossible to book the courts in advance is that they're free to use, so outdoor games are always free.
5) If you have special shoes for artificial grass I highly recommend using them.