Lumpini Park in Bangkok is a welcome green retreat from the hustle and bustle of the traffic that is the Big Mango. It is approximately 142 acres in size (360 rai in Thai). There is an artificial lake, complete with canoes or those annoying pedal boats for rent, (great for kids) several running tracks (which you can also cycle on), some robust outdoor gym equipment, stalls selling drinks, ice cream and food plus several groups of tai chi or exercise groups. (complete with loudspeakers!)
However, there is plenty of shady trees to stop under, and much wildlife, including very large looking monitor lizards! Each year they hold the Terry Fox Fun Run in this park also.
You can get there via car its on Rama IV Road, between Ratchadamri Road and Witthayu Road. BTS station is Sala daeng and the subway has a lumpini station. My favourite option is to follow the green route canal walking/running track from sukhumvit soi 10.