walk/trot past a “flock” of those lawn ornament/windsock-type Canada geese pink flamingos, pinwheel whirly-gigs or string line of colored flags, glittering CDs on string (tied to fence rail; all manner of spooky moving objects. there are infinite variety and they don’t always react the same to each one.
put on/take off rain slicker (DD used to subtly drape slicker over Fjord’s ears & eyes for extra show of how suitable he was. They loved 4-H trail! Equestrian team seemed to be a bit more “breed” oriented - mostly poles, walk-over/lope-overs not nearly so interesting or representative of what might be encountered on a real trail.)
drag a tied up large plastic (or feed) bag of soda pop cans from points A to B
dismount, properly ground tie, walk to a designated spot, return and remount
mount from "“tree stump”
step in hula hoop with front feet and turn on the forehand with feet staying in the hoop
weave thru line of cones or closely-spaced barrels
pick up item (small flag, plastic cup of water, etc) at spot A (off top of barrel) bring to spot B and place where directed. try at different gaits to see how accurately you get a smooth stop at the right locations or trying not to spill the water.
Water box or puddle (make sure bottom is safe, not slippery)
I’ve always thought a 12" to 18" little jump or step-over would make a good obstacle too (but then Fjord did hunters anyhow, so this was almost a “what-ever.” Actually harder to get him to take it seriously.) Also try this dismounted, leading horse over.
Look on-line. There are numerous sites that have patterns of obstacles laid out. Getting some of the patterns done smoothly is far trickier than it looks and just because you can do the individual items doesn’t always make it easy to put it together well. I still play with many of these when my horses (or I) need a break from our normal dressage schools. Keeps them sharp and interested.