Sometimes people have different opinions if it's allowed or not to fly a drone in general, or over the city of Zurich.
This article sums up and translates (in English) the official rules from the Federal Office of Civil Aviation (FOCA) for flying a drone in Switzerland, with a particular emphasis on Zurich (rules from the police of the city of Zurich).
Every DJI drone comes with this flyier in the box, so in theory every drone operator should know it by heart. Here how it looks like:
Your main reference is (in German) is the page Bundesamt für Zivilluftfahrt BAZL - Regeln und allgemeine Fragen zu Drohnen (under "Dokumente" you find also the PDF of the flyer).
The homepage is available English: Federal Office of Civil Aviation (FOCA) - Drones and aircraft models, unluckily the drones section on the English FOCA website is incomplete (what the foca! ) so I'll sum up the most important points for you. But check also the google translated link above.
You can have a look at this YouTube video for dummies, explaining (almost but not everything) I just listed above. Watch it and you may learn a bit of German:
Regeln für den Gebrauch von Drohnen.
What is it meant with "groups of people"? See in Bewilligungen für den Betrieb von Drohnen: "mehrere Dutzend, dicht beieinander stehende Personen" which translated means "several dozen people standing close to each other", which, in numbers means 2 * 12 = 24, please note also the wording "close to each other". So if it's an event, a concert, a restaurant with more than 24 people almost touching each other you can't fly over them, but you can fly from a distance of 100m (horizontal distance) from them. If you are in the mountains and in the nature and there are more than 24 people: in my interpretation of the rules you can still fly between them if they are not all close to each other. But I suggest you to avoid doing so: in general some people knowing these rules may complain and even call the police on you, if that happens how can you prove that these groups of people were not all close to each other? If you film them you may have a chance, but I'd not risk if I were you... this could be actually controversial, so be careful. My personal suggestion: even if you see a few people, try to fly as far as possible from them to avoid any trouble. If somebody comes to you and complains about his privacy, or believes you were spying on him or his family, be polite, explain what you were filming, maybe show him on the screen the subject of your video. If the person still persists it may be a drone hater or a person that is freaking out, in that case try to resolve he problem as quick as possible: explain that you didn't intend to fly that long and that you are about to land in a few minutes anyway. Land as fast as possible... I know it's your right to be there etc. but better avoid any trouble, after all you never know if that person would punch you. People can be crazy, your life is more important than your pride...
If it's not specified officially by a municipality/canton, ski resorts etc. may tell you that it's forbidden to fly over their facility (they may be right if they speak about cablecar facility, or a restaurant, I mean privately owned buildings), but legally their claim has no value over a public space, e.g. if mountain is publically accessible by pedestrians, hikers etc. the official federal laws apply. I read in drone groups people asking to ski resorts if it's allowed to fly there, and of course the answer they got was "no" because of the "groups of people" excuse, but if there are less than 24 people remember that you can stil fly? I even sent an e-mail at FOCA and the guy didn't deny or confirm what I asked but simply told me "Swiss drone rules are quite permissive and easy to understand", so why asking if you can get "no" as answer. Read the official rules, it's as simple as that.
Last but not least: if you fly over some private property and the owner complains, you have to stop. If you filmed it and the guy said you can't, you may have to delete the video or never be allowed to use it (unless you censor the private property). You can still hover your drones a few meters away from the private property and the owner can't say anything about it if you film something else. It's your right to have a drone over public space.
Most of the city of Zurich is inside the CTR (control zone) of the international ZRH airport. You are not allowed to fly closer than 5km from the airport runways, unless you get a permission from the tower. Outside of these 5km the regulations allow us to fly a drone till a maximum altitude of 150m.
Check the Switzerland map layered with drone restrictions to see exactly where is the boarder of those 5km from the runways, here a preview:
If you checked the links and read the official rules and laws linked above, you may have noticed that in several places it's mentioned "between 0.5 and 30kg".
See the legal basis at DETEC Ordinance on Special Category Aircraft » Section 7 » Art. 171: Restrictions on model aircraft » paragraph 2
I even asked FOCA and the guy answered me "you are right, for drones, toy drones etc. the restrictions apply only between 0.5 and 30kg. So in theory it's not explicitely forbidden, but not recommended to fly too close to an airport or over groups of people."
I would generally stay away from groups of people, as people tend to piss off easily. For the airport: as the guy from FOCA said, I'd suggest to be extra careful. If you have a really cool spot where you wanna fly, within 5km from an airport (but I recommend at least 2km away), try to stay at a low altitude (e.g. at the same altitude of buildings and natural obstacles, like trees...), and try to avoid publishing the video (if filming) to avoid a shitstorm. Or at least specify in the video that you used a drone weighing less than 500g. If the police comes to check you'll have to prove it.
It takes only one accident for the rules to become more restrictive and to create super restrictive laws like in Canada and recently also in Germany... so fly safe and avoid any risk.