Hay bales are always an easy option. You can make a narrow zigzag-like "street" for the horses to walk through (bales on each side), or just set them up as jumps.
Find a fallen tree in the nearest woods and make a jump out of that.
Flower boxes are also easy to work with and can be nice training tools for despooking, especially if you have taller flowers or grasses in them that move in the wind.
And you can use regular (ground) poles and scatter them in an irregular fashion so that the horse really has to navigate stepping through and over them.
I'm sure if you simply google "cross country jumps" you will get a whole bunch of websites giving you many more ideas. Have fun designing your course!
It's easy to build your self a bridge out of 2x4s. It doesn't have to be high off the ground. Just enough to give it that hollow sound. Many of the forest service bridges we cross are only 2' wide and various lengths. To add a little more spook factor to it, Build one that rocks 3-4" as you cross the half way point
Our biggest obsticles are just rocks and boulders. The horses have to learn to pick their way through. If you have any rocks on your property, maybe you could collect a few and concentrate them in an area with trail through them. http://i130.photobucket.com/albums/p...8/SANY0044.jpg
My parellified neighbor's fifty acres is crammed with creative obstacles she put up for a play day. None of them really got much of a spook out of my horses, but they had been through a lot of "natural" hazards out in the real world (although mylar helium balloons aint that natural, out on a dirt road was where we happend to first meet that particular booger.)
Here's some of the things my neighbor has used:
Lots and lots and lots of junk tires. Down flat to create a maze, up on their edges to make a narrow place to squeeze through. Giant bulldozer junk tires filled with sand or dirt make platforms.
Everyday oak pallets make dandy little bridges/platforms with a good heavy sheet of plywood nailed to them.
A gazillion whirlygigs and sparkly windcatcher thingys she bought at the Dollar Tree. Ditto little American flags and assorted banners and chinese lanterns. These are stuck on posts and hung from trees. They whirl, twirl, and some (little plastic wing flapping birds and such) make a racket.
Milk jugs spray painted in screaming fluorescent colors and partially filled with sand or pebbles. Used to make narrow "bridges" and also hung from trees. They bounce and rattle when brushed up against.
Aluminum foil pie pans, also hung from trees, a cluster of them together. They move in the wind, flash in the sun, and go clank.
Shower curtains hung from trees. Make horsie think he's in a Japanese restaurant.
Pool noodles threaded onto rope and hung from trees, or scarier, nailed to uprights and forming a sort of turnstile horse has to push through.
Finally, dummies made from old clothes stuffed with newspaper or more old clothes. Spooky immobile silent person lurking on the trail.
Of course, I tend to think all this dreck detracts from the considerable beauty of the woods, but nobody's forcing me to ride on her trails so I grin and bear it, and so do my horses.