Generally, in winter, it might be a nice day with +8C, sun, and you enjoy outside, or freezing cold, windy, feeling like -15C, and you'll definitely not want to spend much time outside. So, if the weather is nice, you can still do some outside activities, while most shopping will be closed on a Sunday, the Supermarkets/Shops in the Main Railway Station are still open. Restaurants of course too. Besides that, much less people on a Sunday in Zurich Downtown. There will be even some of the big ships travelling on Lake Zurich (starting from Bürkliplatz) - if the weather is good, this is a nice option, there are short roundtrips which don't cost an arm and a leg. Besides that, there is couple other nice stuff to do in winter outdoor, like taking the cable up to Felsenegg, which could be combined taking a train to Adliswil. Often, when it's cold and grey "down" around Lake Zurich, the hills up Uetliberg/Felsenegg are a nice place to go, with sun and snow. For such a trip, you could buy a day card for the whole Kanton Zurich / ZVV 9-o'clock pass, it's 24 CHF full fare for adults, so not that much more than the ticket just for Zurich itself, and it includes all public transport within Kanton Zurich and the lake area. (Rapperswil, if you take the boat for the big 3-4 hour trip)

Another idea is going for a outdoor hot Spring, like the one in Zurzach.

Anyway, those for the good weather ideas in winter.. if it's cold, wet, grey, there isn't really that much to do on a sunday. Well, museums are open, and there are plenty of them, plus there are many cinemas in Switzerland (but overpriced!) which are running movies in English (German&french subs only) - but that won't kill 16 hours, and most museums will make you feeling tired after a few hours.. one idea, where you can still take a swim on a crap day would be to visit the recently opened Thermalbad Zürich: http://www.thermalbad-zuerich.ch/

I've never been there, but it sounds pretty cool, you can bath within an old beer brewing cellar (as well as outside) and it's still half-affordable with 32 CHF.. (Switzerland is expensive) - other things, if you like water rides, obviously the Alpamare in Pfäffikon SZ, even more expensive and no Thermal bath but rather a huge amount of rapid rides.. there is some relaxing part as well, but it's mostly for teenagers, kind of a 6-flags in water.

If you want some "hot" indoor water/swimming combination, maybe going to Water World Wallisellen might be a good alternative. Huge 50m pool, some jumping areas, one 75m run (which is not too bad ) and much more affordable at 7 CHF - it's a well-run public bath with a lot of attractions, and even in winter without the outdoor part still pretty big.

Of course, if you are really adventurous, you could also catch a train to the mountains, rent some ski's or sledges, and go have fun.. those things I've suggested are those you could take in the close neighbourhood of Zurich under all circumstances (for example, if there is a lot of snow, there is a sledge run right into Zurich, but this depends of course on the amount of snow available..)