Ping-pong balls spread across a desert will reflect sunlight back into the sky, and will form an insulating layer keeping soil temperatures moderate. Dew and rain will trickle down between the balls, and plants can sprout in the sunlight coming through the gaps. The balls will reduce duststorms when the wind blows, soften the blow of raindrops hitting parched soil, and reduce erosion.
Ping-pong balls spread across farmland will always rest on the surface of the soil, even if ploughed under - they'll soon pop up and reflect sun from bare fields. Soil moisture loss from evapouration will be reduced, and temperature extremes will be less pronounced. Frost will be less of a danger to sprouts. Crops will grow up between the gaps, and their leaves will get extra insolation from the light reflected from the balls - useful when we exploit fields farther north than before, with shorter growing seasons.
Ping-pong balls spread across dark rooftops will reflect sunlight and provide extra insulation. Monsoon rains will no longer pound noisily on tin roofs, keeping people awake and irritable all night.
Ping-pong balls spread across glaciers will reflect light and provide insulation, reducing rapid melt during summer. Some ping-pong balls may become entrapped in the glacier after wet snowfall, but most will pop to the surface the next spring.
Ping-pong balls spread across lakes, ponds, and reservoirs will keep the water cooler and reduce evapourative losses. Algal blooms from farmland fertilizer runoff will be reduced. There will still be some penetration of sunlight to littoral plants, and oxygen will still reach the water surface to support aquatic fauna.
Ping-pong balls spread across oceans will slow the transfer of heat from tropical water surfaces to the air, reducing the size and frequency of tropical cyclones / hurricanes. The atmosphere will have slightly less water vapour (itself a greenhouse gas), offsetting the impact of carbon dioxide. The balls would be big enough for most sea life to not swallow them; if swallowed by whales or larger animals (if not chewed first), they would pass through with some discomfort. Phytoplankton would still thrive in the gaps between the balls, and barnacles, corals, and other sedentary life could grow on them.
Now, this is just a thought experiment. There is not enough readily-extractable oil to make the plastic to make that many ping-pong balls, and the cost would be huge. But other materials would work, too. Crushed dolomite gravel (like white limestone) could be applied to land surfaces and roofs, for example.
After harvesting coconuts, farmers are left with huge mounds of coir (the hairy fibres that formed the husk). The longer fibres can be used for making ropes and mats, but most of the rest is waste. Coir floats, and lasts for a long time without breaking down (it's still washing up on the shores of Nova Scotia centuries after being used in cofferdam construction on Oak Island, for example). If we bleach it, and then dump it in a city reservoir, it would cover the surface, keep the water cool, and reduce water loss from evapouration.