"A Sunday in December" is not precise enough to give a proper answer, as the reply will depend on which Sunday...
It’s 16th Dec
It's a pity as you will miss this: http://www.1602.ch/. Regular Sunday mornings are very quiet in Geneva. So a brunch might be the best option. I like http://www.lechevalblanc.ch/ in Carouge (UberX would cost about CHF 25-30 to get there - by bus (10) and tram (18, change at "Cornavin"), it would take you about 1/2 an hour.
Brunch by the lake, I am only aware of the hotels. But may-be something new opened...
In the Old Town, there is a place where you could have a nice breakfast (not brunch): https://www.tripadvisor.com/Restaurant_Revie…Edited: 4:33 am, November 03, 2017
Thank you. May I enquire how can we get to "Old Town" from the airport area? and is there another name for "Old Town"?
No other name. It is a hill on which lies the Cathedral. If you want to introduce an address on a map, try one of these: Rue de l'Hotel-de-Ville (i.e. City Hall street) or Place du Bourg-de-Four or Grand Rue.
Bus 10 will bring you down from the airport to the bottom of the hill (stop either at "Molard" or at "Rive", the final one).
Thank you very much!