There are tons of companies that will sell you stuff to put on your battlefield or you can find your own stuff. I go both ways. I have purchased lots of stuff over the years, but I also like to make my own stuff from time to time.
Recently, I had a friend give me some stuff from the company he works for. It is some kind of medical thing-a-ma-jig. I don't really know what it was supposed to be, but it didn't meet specs so was being thrown away. He gave it to me since he knows how much I like stuff.
To start with I just painted these white. That was easier said than done. These are small and very light. So when you spray them, they just fall over and roll away. So I ran a couple of strips of masking tape out on a board with the sticky side up. I tucked the ends under so the tape was attached to my paint board. Then I stuck these barrels to the tape and then sprayed them. After they dried I flipped them over and painted the other side.
I started by painting a few blue and then washing them with a blue ink. I decided that was too much work so thought I would just see how a wash alone looked. I think it turned out well and did a variety of colors. The dark blue ones in the back on the left were my first attempt. The others are just washes.
These look good with 15mm guys and would also work with 28mm guys. Here you can see some Earth Force Marines (from RebelMinis) taking up cover behind a few barrels.
These might also work for 6mm guys but they will be a bit big so you might want to keep them in an industrial complex. I might convert some into some kind of industrial storage containers for 6mm. Here are a few GW Eldar stands taking up position next to a few barrels to protect them from some invading force.
I used pretty bright colors to paint this batch, but you could go with more muted tones depending on the look you want.
What do you all use for the "stuff" on your battlefield? Do you buy items or do you re-purpose items?