If you build obsticles that appear like those you will find in the open country, They would be pretty vandal proof.
Steep down hills or up hill climbs
Wooden Bridges like this one over a boggy area
Put up a section of split rail fence. You can practice side passing over and picking up a hat or coat. We call this fence the Knee Knocker, because of how close it comes to our knees as we trott along side of it.
Tight spots. Here we are between two rock walls. But it could be a wall and a fence just something that ask the horse to trust and pass thru
A gate is a good idea. But I really don't see many gates out in the open range. If I do see a gate, they are usually barb wire and not something that you can open from horse back.
We ask our horses to climb up on objects Something like this would be hard to vandalize. We also make the horse go off the high side so they learn to climb up and off ledges
And we ask them to cross natural stone ledges
And of course Rocky trails are a given
Crossing blow down or dead fall timber
Tunnels or places they have to enter.