Custom barriers are basically anything that isn't under the "barrier" section. Try using chain mesh in construction, or fences in props (a section construction). For a natural look, use rocks. You can also add detail to the existing barrier sections.

In the barrier section, there is something called null barrier. Its intended use was to be there when you don't need a barrier, but if you're in sandbox you can use it differently. Click on setting, then sandbox, then animal settings. You should see an option to turn off escape. Now you can use null barrier to make a national park!

When creating really anything, you should detail it and this applies to planet zoo as well. For nature, use bushes flower, etc to create undergrowth and beautify your scene. With construction, use a section called Wall Decorations (I think). Place these on your walls.

If you want to create something that's old, there are a few things you should do. First, cover it with plants. There is ivy which is quite a good option. If you have the Europe Pack, there are some great wall details for your overgrown building.

Do you wish that you could place staff buildings on a non-staff path without guests running into them. With this trick, now you can! In Bins, Benches, and Security there is a sign that prevents guests from walking on paths. It is shaped like a hexagon with a hand on it.

Let's say you want a dinosaur in your zoo. How are you going to make that? In Construction, there is a section called shapes. You can use the 3-D shapes to make a dinosaur. It won't walk around, but it will look good!