"Easy" is often the hardest bit.
Sure, AK offeres up some "one pot" solutions, but they can be spendy if a person leans to thrifty (or "cheap").
Now, one way to get "rubble" trash is to take a Surform rasp to cork sheeting, which will give some random chunks. Cork just loves paint, and cares not a whit if it's the cheap poster paint fro mthe art store (or high-dollar modeling paint). It will take paint washes quite nicely, too.
Now, for inspriation in 1/87, a person could take to watching Kathy Mallet or Luke Cowen videos on YT.
Now, in fairness, Luke's often seem like advertizing video for his sponsors--but, if you are being paid to promote stuff, you promote it.
Another interesting diorama builder, who does include trash and the like, is Thomas at Laser Creation World (which, also, can seem like ad copy--but he has some real art-school paint techniques to show, too).
Your Mileage May Vary