I would like to stay at Giethoorn for one night; I want to store my baggage at Amsterdam Zuid station. Are there any baggage lockers one can use?
Here is a more recent answer (from mid 2014) from an NS employee basically confirming there are no lockers at Amsterdam Zuid. (Sorry, only in Dutch.) She also mentions that unfortunately there are no alternatives at this particular station. I.e. at some stations the local bike rental companies offer locker storage.
To confirm this information is still correct, you can check out the official info page on station Amsterdam Zuid, which also doesn't list any locker facilities.
Your best (read: closest, if time allows) bet may be to catch a metro to station Amsterdam Amstel (4 stops off the top of my head), which does have storage lockers. Note that this isn't a staffed facility, but rather secured electronically. Lockers come in two different sizes, payment is per 24hrs and the rate goes up with the rental period, which is limited to 3 days. More details can be found here.
A quick Google search provided the answer :
So there don't seem to be any.
And the most recent answer (2-3-2017) comes from the official ns-app 'reisplanner' or the website Still no lockers though.