This is a must do.
When you're in Evian or Thonon-les-Bains, make a day-trip to Yvoire. A return ticket costs you 25 € but it's really worth it on a sunny day.
The boat takes you slowly along the lakeside .
Beautiful scenery is waiting for you.
And then you arrive in the colourful Yvoire harbour.
If you are not traveling by car, you can take a ferry from Geneva to Yvoire, from the information I have it currently costs 36.20 CHF for 2nd class, 49.40 in 1st class and the journey takes an hour or more each way depending on the type of boat, it's almost 2 hours via paddle steamer.
I assume it stops within walking distance of the medieval town but you might want to check that in advance, the people at the docks in Geneva are very helpful.