Suggestions for 2 days in Switzerland

Welcome atrekker! If you're looking for a stay in the Alps, the Bernese Oberland is definitely recommendable. It's easily reachable from Basel and Zurich, so you could spend two nights there. Consider Interlaken or a nearby town. If you appreciate scenic train trips, you should travel to Zurich via Meiringen, the Brünig Pass and Lucerne. Furthermore, you can make a boat trip over lake Thun, visit one of the famous viewing points such as the Jungfraujoch or the Schilthorn, and indeed make some hikes. Routes at higher altitude may already by closed down due to snow, but there are lots of routes that are still accessible this time of the year. This overview might help.

As for a rail pass: if you need one, and which one, ultimately depends on your plans. If you'd only need tickets from Basel to Interlaken and from Interlaken to Zurich Airport, regular tickets would be best. But I assume you will be traveling more than just those trips. The Swiss Half Fare Card is probably the best solution in that case.