If you look on the ACTHA site, under "our rides" there is a whole section describing the different obstacles, what they are judged on, and even videos showing good execution.
One obstacle a friend has that I really like is a blue tarp, with dirt, river rocks, a stump/log or two, set up so that the only "blue" that shows thru looks like a little creek. Sometimes she'll hang a hide over the stump--or (practicing for the ACTHA "Spooktacular" last weekend) a pumpkin or ghost!
I started my bridge work with just a sheet of 3/4 inch plywood, then built it up on 4X6s. To make the teeter tooter, I just add a fence post in the middle.
Just exposing your horse to lots of strange things is a plus (our last ride had a giant flourescent stuffed cobra!). And practice all your good basic skills: backing, sidepass, gates, etc.
Good luck, and more importantly, have fun!