You could work on walking up to a mail box and taking letters out of it and putting them back.
A Saddle Club down the road from us has a big round bale feeder turned on it's side with streamers tied all around the top. Just make sure you block it so it won't roll when you go through it.
When you say you have worked your horse with a tarp, did you carry it on the horse or ride over it?
You can walk your horse over a wooden bridge, the noise will sometimes get to them. You caould make one out of railroad ties and plywood.
Opening and closing gates while mounted.
Stepping over logs.
All the obstacle challenges I have seen the horses need to do it willingly and with little to no hesitation.
That is all I can think of right now,but I will keep thinking. Good Luck!!
SC