Needed a SIM upon landing in Geneva and was told that the only store selling SIMs was the post office. They were unable to sell me one (something was broken) so I picked up a "Yallo" SIM card at the Swiss Post Office in Zurich.

For 10 CHF, I got a SIM with 200? MB of data and 20 CHF of credit. Had to show my passport, and the process was slow, as they needed stuff not on the passport, like current address.

You can only apply the 20 CHF credit after the first batch of data runs out. Then you can choose roaming plans, or just data, in various increments. I texted "SURF1000" to the same SMS address that had been sending me ads and notifications, and 15 CHF of my free 20 CHF was used to allocate a 1GB block.

Yallo provided 4G coverage in Zurich, Geneva, Lausanne, and on the train between those places, sometime dipping to 3G. I'd use them again.