Beetroot

Foxtail millet, available in pet shops can help. Rub the grains into a waggon, apply thinned white glue..... et voil?

Apples

There once was a wagon-set with an apple load available from Roco. The apples can realtiveley easy be scratchbuilt using millet or other grain food. Paint a piece of wood or styrofaom that fits snugly into the wagon with white glue and sprinkle on the material. After letting it dry, stipple on various shades of red and green. As afr as scale goes, these apples my seem a bit large but alternatively, only colored sand will do the job.

Pipes

Cut straws to length, paint them silver, black, or a rusty brown. Cut matches length-wise to use them as rest for the "pipes". Glue the completed assembly onto your car.

Wood stacks

... can easily be made from the wooden stirring sticks provided by McD.... The scrap can be used as woodchip load. Matches, length-wise spli, also serve well for this kind of load.

Woodchips

Sawdust is the material we want. Sift it, so that only the dust remains as load. Now model the surface of a suitable piece of styrofoam to a somewhat "hilly" surface, apply a thick layer of white glue, and sprinkle on the sawdust. Done

Junk

basically, everything from your scrapbox can be used as junk load. Maybe you need a hammer to shredder the parts a bit further. Apply some plastic cement to the car´s floor, decorate the parts, paint them in various shades of rust, and you´re done.

Slag

Smokers have a big advantage for this load :-)! Cigarette- or cigar ashes glued to a fitting piece of styrofoam yields a superb slag load. Non-smokers also have their chance:
Clay granules, usually used as alternative for flowerpot earth, also lends itself to this job. Shredder it with a hammer, glue it to a piece of styrofoam and you have your load.