I love this idea and have several images floating around my head. Broken bricks might work, plastic bottles, glass pieces (maybe), the plastic rings that go on a six pack of cans (but only if you have fish small enough to fit through the rings, otherwise you could have big problems).Also, the water should look dirty. I'd paint the background to be a mix of greens and browns, perhaps fading from brown at the bottom to green towards the top. I'd also let algae grow as it wishes. For substrate, I'd use a mix of sand and medium-ish size rocks (2-3" range). Also, driftwood and plants mixed in with the trash would make it look more authentic, IMO. Of course, this is all what I'd do if I were doing the tank. You might want something else. Also, if it were me, I wouldn't use toy miniatures; I'm envisioning a tank that's a cross-section of something you might find out in an urban area.
Also, I believe somebody on here made little nuclear waste barrels for a tank for their dad. I can't remember who. I'll see if I can find it.