If you're only here for a few days and want to experience the best of Switzerland then the best is what comes free - the scenery and the lovely cities and towns. If you're going to the auto show then that leaves you three days which is just long enough to experience a bit of the French speaking area and a bit of the German speaking area. In the German speaking part I would choose one city and for me that would be Zurich - there's more to see and do there. There are frequent intercity train services between Geneva and Zurich and if you start early and come back late you can make a day trip of it.
Here's the website of Swiss Railways which will allow you to plan your trip. Train is by far the best way to see the country:
http://www.sbb.ch/en/
Around Geneva you should focus on the lake. In addition to looking round Geneva (it's not big and half a day would let you see all there is to see) I'd choose two towns to visit along the lake - Nyon and Vevey. (If time is limited don't bother with Montreux - Nyon and Vevey are much prettier) but if you have time Chateau Chillon near Montreux is worth a visit. There's a bus service between Montreux and Chillon which runs every 10 minutes. Again, there are half hourly train services which connect Geneva - Nyon- Vevey-Montreux. It's the same line so one return ticket will do the job and you can hop on and off where you want. As well as seeing two of the most attractive old towns on the lake you'll see some of the most stunning scenery in the country - lake, mountains (which in March will still be snow capped), vineyards, castles etc. If you're up for lunch you should try filets de perches and french fries - it's the signature dish of the area.
The train trip to Zurich if you go via Fribourg will take you through some lovely countryside between Lausanne and Fribourg - all rolling hills, pastures, old farms, castles etc. The leg between Bern and Zurich is nothing to write home about but you flash through it anyway. Zurich will take a full day to walk around the old town, see the lake, have a nice lunch, see the cathedral etc. You should eat some rösti and bratwurst whilst you're up there.
If you don't have time to visit Zurich then Bern is a little closer and a nice town to walk around.
If you are going back to Geneva from Zurich then go back via Neuchâtel. It's a different route from the one via Bern and you'll see different scenery including the lovely lake Neuchâtel.
I could write an essay on what to see but I'll stop at the big picture and suggest but buy a guide book to see what interests you.
Hope you have a great time.