I am living in Switzerland right now, and I think overall Switzerland is a great place to visit. You cant really go wrong. If you like more open people and a little warmer temp (especially in may) Lugano, Locarno, or anywhere in Ticino, the Italian part of Switzerland, is bound to be nice. Also, Grindelwald is pretty amazing as well as Zermatt, which has the infamous Matterhorn. And Geneva, in the French part, is also very very beautiful. And Appenzell, a quieter place in the east of Switzerland that makes great cheese, is also so amazing. It really depends on what you are looking to see, and what you like. Also, I think May is a great time to go because it is spring and when the fog isn't so much present and it is warmer.
oh! and make sure you bring a lot of money, Switzerland is very expensive.
And Switzerland has an AMAZING public transportation system. You could do a whole tour of the country by train, and it is not too expensive... cheaper than by car and easier.
So yeah, there ya go, I hope I have helped! :)