One of the best attractions in Geneva is definitely and will always be the Leman Lake (or Lake Geneva). I can only talk for myself, but I know that most of the tourists and locals in Geneva, love to do as many activities as possible including this massive bulk of fresh water.
The cheapest and coolest way to go from one side to the other in Geneva are the famous Mouettes (Seaguls). They are included in the monthly subscription of your TPG and if you are just for visit and staying in a hotel you have them for free too (Check the how to move in Geneva post)
There are 4 Mouettes, these are their itineraries:
M1: Molard (Shopping square in front of Lacustre) to Pâquis (In front of Kempinski)
M2: Pâquis to Eaux-Vives (Next to the Water Fountain)
M3: Pâquis to Genève-Plage (Nice Pool and chiiling during sumer)
M4: Genève-Plage to De Chateaubriand (Perle du Lac)
They are super fast and you can take great pictures when you are in them!
Cost? 2.00 Saut de Puce!
Jump into the lake and cross in a beautiful yellow SeaGul!