Gojek, Grab, Bajay or metered Bluebird taxi would each cost less than 40,000 rupiah from central Jakarta, but I think you may be sorely disappointed at the very small size of the park and the restrictions on walking on the grass there, of which there is very little anyway. There are a couple of nice fountains, but the park is surrounded by roads and traffic - and is hardly a respite from Jakarta's concrete jungle.

Taman Suropati is not as crowded as taman Menteng. The crowd comes and goes. Best timing to come is weekends around 3pm but it is still nice at anytime of the day really. Sometimes you can see people sitting around or play some instruments or doing some sports.