The beautiful Cathédrale de St-Pierre is situated in the heart of Geneva's old town (that lays on a hill) and was built between 1150 and 1230. Many adjustments have been made since then'; statues and other decorations where removed during the Calvin years (16th century), and mid 18th century the facade was reconstructed in neo-classical style. You can climb the tower: after about 157 steps you'll have beautiful views of the city and the lake from the north-side of the cathedral.
If you have a fear of heights, you can visit the archeological excavations in the museum underneath the Saint Peter. Where you get to see de remains of the churches that used to stand in the same spot. The oldest ones date from 350 BC.
Address: Place du Bourg-de-Four 24, 1204 Geneva