I live right next to the Parc Trembley. It has a shallow pool for children in the summer which is free, along with free lawn chairs. There are barbecue areas as well as a fitness area. Great for walking, chilling out, sunbathing, there are also ping-pong tables down by the school.
This nice park is located between the Servette Area and the Petit-Saconnex. It's one of the biggest park of the right bank. A bit overcrowded during the weekends, but otherwise quite peaceful, with some very big trees.