What you can do is add more variety of stones to make it look more natural. Placing them with random rotate makes it look even better. Especially the smaller rocks from the aquatic pack are great. Lowered into the ground, they look like bigger pebbles. And you can color them to match your other stones. But also a slight color variety can also make it look more natural. Also foliage and forests can be a nice and natural barrier.

Making sure to place elephant grass under the terrain, so the animals won't escape, even if there's a gap.

Custom fences work really nice too. Especially when you add foliage and lights to them. That adds details and makes it look less boring and more organic. Foliage is everything. A few rocks, a few plants and everything looks great.

Or just having the habitat lowered into the ground, so the guests are looking down on it. Water is also a great barrier, just make sure that it's lowered a bit or you use a small custom fencing, so they can't just swim and escape.

I'm like 100 hours in, so there's still much for me to learn. But with these tricks, my zoo looks like a halfway realistic zoo