This year, you'll need to go pretty high to play with snow. It's been a particularly mild winter, and many of the ski pistes at lower elevations in Germany and Austria have already closed. But by contrast, that'll mean the good hiking trails will open earlier, although May may still be too early.
I haven't visited, but my brother enjoyed the Lindt factory tour in nearby Kilchberg. His souvenir was a box of chocolate that he himself had made.
May could be too early, but the high-altitude hiking trails in Flumserberg, about 40 minutes southesast of Zürich, are particularly family friendly. Once you ascend the lift to the near the summit, there's several kilometers of nearly flat trails. And plenty of playgrounds. But even with the mild weather this year, conditions still may not be suitable in May.