What era? Any time in the ‘modern’ era (say 1970s to today), most dumpsters used by commercial/retail service would be loaded primarily w/ plastic garbage bags full of trash, and cardboard (from boxes etc)…
I have made some ‘plastic bags’ from molding clay (roll into a cylinder, chop into, say, 2 scale foot sections, mush down into a kind of ball, add a bit more clay to represent the tied off top part, paint gloss/semi gloss black or white (there seem to be few other colors for general use - there are blue and red bags, but they tend to be used in specialized service). Cardboard - pieces of cardstock or styrene squares, paint flat light brown/tan or white.
Other bits of garbage can be added (little dried up crumbs of filler putty representing crumpled balls of paper, or if slightly bigger and w/ a dollop of red/yellow paint, they can represent a crumpled up fast-food bag), snippets of clear glass rod as bottles, etc., and maybe some funky stuff (an old chair, tv, bucket) - don’t overdo it though.
Edit: Cause I can’t stop talking trash…Industrial concerns are different, of course, although they have dumpsters w/ the usual bagged trash, they may have a separate dumpster for cardboard and paper waste, and (perhaps the most interesting) a dumpster for recycled metals - coils of wire, metal sheets with stock punched out, bar & angle pieces, etc. Pallets also seem to inhabit dumpsters (they see hard use, and so get chucked often). And you can throw in some branches and greenery to indicate recent landscaping around the facility.
If modeling a restaurant, you can add a steel drum to hold grease from the broilers/friers - relatively clean looking but definitely use gloss black acrylics (artist paints) to represent grease around the lid.
One thing, I would caution against going overboard w/ the ‘cute’ scenes, such as treasure chests, dead bodies, living palm trees, a full televi