Two simple measures of park access are the total area of parks in a city and the average distance of residences from a park. You can divide the former by population to get a normalized stat that can be compared across cities or tracked over time, and you can look at various stats on the latter such as the percent of households within 10 minutes of a park. Here are a few more ideas from a guy in Singapore.

• length of walking and/or cycling trails
• length of waterfronts (not sure exactly how this was defined, if it included Singapore’s concrete drainage channels in addition to oceanfront, lakes and ponds)
• area of dense vegetation within parks
• “supply of of park area to residential buildings” based on a decay factor (some sort of weighted average I suppose based on how much people of willing to travel – paper here)

This all makes some sense to me. I might add some measure of tree canopy. None of this does anything about the weather in Singapore. If you want to enjoy parks there, I recommend getting up very very early. Then take a nice long afternoon nap, stop by the pool if you have access to one (but if you are light skinned realize you are at the equator and you still need sunscreen late in the afternoon), and go enjoy the more urban amenities (like food, very large bottles of beer which are meant to be shared, and a variety of less family friendly entertainments I have only heard about) after dark, which is around 7 p.m. year round. One thing about Singapore is it is safe to be out at any time of night, and street food is available all night.