For a smaller park than Lumpini, and one close the Sukhumvit area, Benjakiti Park is the one for you. You'll find it by the Emporium shopping mall, with its green lawns, lake with fountain, and its many shaded spots, and running and cycling pathways. 

The busiest time at Benjakiti Park is towards late evening, when the cities populace come our for a stroll, exercise, or simply to take in the greenery. Sunset over the lake is spectacular and when the vibrant hue of flowers and the lushness of the trees are added to it, the place becomes ethereal. Take a picnic, you won't be disappointed.