Almost anywhere in Switzerland is within 2.5 hours from Zurich or from border to border for that matter. It takes about an hour and a half to reach St. Gallen by train from Zurich. Remembering to buy some food and drink at the COOP opposite Zurich main train station before setting off. At trainstations prices are 3 times as expensive!
Its a nice journey to St. Gallen and time passes quickly. St. Gallen is a medium sized town with plenty to do and see. For food there is a number of places to eat most expensive so I suggest heading out from the train station to the Migros and COOP restaurants, you can see their signs from the station. Theres loads of small streets and squares full of small shops and interesting for a couple of hours at least. A good way to see St. Gallen is the Trolley bus system and the Trogenbahn Tram. The tram used to be painted bright orange and goes right out into the countryside climbing upwards towards Trogen. The orange colour is now a thing of the past and the quaint old trams are now modern red trams. Another trip further away from St. Gallen is to go on the Appenzell Railway all the way to Appenzell country. You get the train at the Western end of the main railway station in its own little station. Having reached Appenzell Town its another town to spend a couple of hours wandering around. Note that the food shops close on Sunday and Monday afternoon leaving you reliant on the rather expensive restaurants. From the town you can return to St. Gallen or continue around Switzerland by coming back on a number of different routes. You can get a train to Altstatten and this short trip is another one of Switzerlands scenic railway journeys where the journey is better than the destination. the train has to slowly rise and then descend down to Altstatten using 'rack' assistance and the train rarely exceeds 10 kph. You can in fact roll the windows down and almost touch the cows in the fields outside. In fact some cows seem to be coming towards the train hoping to get a pet.