Party is by now planned in a boring, corporate environment. But while I do not need it anymore at the moment, I believe it would be interesting to keep the thread open...
For a more laid back, family-summer scene - there is the Saal am See in Wollishofen. It is right on the lake, with a little grassy park, a BBQ and a playground for the kids...
Another good place is Artecotta (not sure of the spelling) on Limmatstrasse. We went there for a friends birthday a month ago, he had caterers come in and we lit fires in pots inside to stay a bit warm, a really nice space. It is directly on the Limmat so would be perfect in Summer. I was a bit surprised leaving at 1.00 or so at all the activity on the steet outside but it does add some extra colour to your evening!!
is that the one next to Lidl, the music bar? If so ive walked past it a couple of times on my way shopping now and keep meaning to pop my head in to have a gander.
Ideally I'm looking for somewhere that serves drinks but will allow me to do the apero for my clients. I did one at a hotel in Zug and they gave me the space for free as my clients were very thirsty. It doesn't have to be a completely private room, a cordoned off area in a larger bar would also work. Somewhere between the HB and Viadukt would be perfect.
I think I have found the perfect place for your thing when I was looking myself: 3 mins from HB in the direction of the viaduct - you bring finger food, they serve drinks. No rent, but 1200 CHF consumption with normal Zurich prices:
there is a room above one of the sihlcity restos, think it's called papiersaal. No idea what it would cost but I have been at a party there and it was really good