I guess it would depend on what "type" of obstacles you are referring to. Since my experience is primarily in ADS shows, yes, you can pretty much use whatever you want for an obstacle, less live animals (no chickens in cages or goats on a rope to drive past). However, that depends on the "class". Cones courses obviously use, well...cones. Town & Country courses are required by the definition of the class to use at least 30% of objects that you might find on a country drive, i.e. mailboxes, fences, trees, last year one show had a chair with an umbrella, etc. Gambler's Choice courses are set up more like breed show obstacle courses, where there is actual "stuff" to drive around, under, and through. It might include fences, flags, water, etc. Here is a site that has photos of a number of different styles of obstacles:
http://www.carriageclassic.com/classes.htm
Here is another one at a local open show:
http://picasaweb.google.com/janelicht/3Gaits4thOfJulyShow?authkey=Gv1sRgCLPskarV9aOZqwE&feat=directlink#
Here are obstacles and cones at a CDE:
http://picasaweb.google.com/janelicht/HickoryKnollCDE2008?feat=directlink#
At home, we use PVC pipes on the ground, cones, trees, bushes, old rugs, large old pieces of plywood (not peeling, but not good enough to build anything with), old 2x4s, old garden hose in various shapes (circle, spiral, etc.), plastic tarps, bubble wrap, pipe insulation, water puddles, dinner bell on the fence to ring, concrete forming tubes stood upright, fabric strips hanging from the frame of the barn door, anything that is safe. (When our son was little, I used to have him drive is little ride-on John Deere Gator around my horse. The horses were far enough along in training to be ready for it. Now he rides his bike around them.)
We vary our driving practice from rail work to obstacle work. Once the horse goes well round and round (rail work), then we introduce more complicated obstacles. It really keeps their mind fresh and they seem to enjoy their work more. We introduce slowly and let the horse pick their way around the obstacles, and ask for speed after they have "gotten" it. (We use cones right away with line driving.) The idea is that you find stuff that the horse might "question" and expose them to it until they quit questioning you anymore and just trust you. That being said, you have to make sure that whatever obstacles you choose, the horse can't get hurt by them or you will have messed up that trust.
Myrna