Why ravens play with garbage?
Ravens and other corvids are among the most intelligent birds. Of note, crows and ravens are known for their ability to solve problems that require tool use, memory, and delayed rewards (patience).

Many animals play, and ravens and crows are no exception. (A study entirely on raven nestling play.) The evolutionary purpose of play activity is thought to improve coordination/dexterity, provide exercise, strengthen muscles (such as wing-flapping by young birds that have yet to fledge), and develop social skills.

As to what type of objects ravens find most interesting to play with, I'm unsure of a definitive answer. Some birds, particularly the satin bowerbird, even collect colorful objects. My hypothesis is that the choice of object is likely due to simple availability (discarded) and conspicuous nature (color). In other words, ravens more easily find such objects because they are colorful, which in turn are frequently human-manufactured.