Designing inside the house, and outside, assumes the same degree of creativity. Keep in mind, the main thing that changes are the assets your designing with. Both areas must be divided into sections. Inside the house these sections are called rooms. Outside the house, you can think of them as being themes.
..... For example. In your yard area you may have a fair amount of empty space in one of your corners. This could be perfect for a "camping" theme. Place a tent or two, seating area, bon-fire, and other camping decor, etc. Add your foilage/trees as required to improve that theme.
..... If you have enough yard space for yet another theme, toss one together, using foilage to decorate just that theme.
Lastly: When you're done, you have a yard of themes, which you then connect using paths, and other decor. This will make the rest of the yard area blend into the themes.
Important: Don't cram, or force themes to fit. When done, you'll want an open environment that doesn't look like a junk yard of items all over the place. Be semi-scarce.
Also important. Rooms in the house are created horizontally. Themes outside the house are created (sometimes) vertically. Meaning, when creating the camping theme in the corner, raise the land to give it it's own camping mound.... leaving an area for a path up to it from the rest of the yard when you connect your themes.
One theme at a time, your exterior design will create itself.
Good luck.