Little Extra Details
Little extra details are what can be the most fun part for anyone looking at a layout. While a detailed planned out scene may be interesting, it's little extras that can surprise people.
This black bear is often overlooked when people initially see the layout. Once they find it they can't believe they overlooked it and it's something they never expected to see. The bears den is located under the bridge.
Here is a cabin set back in the corner of the layout in the middle of the woods. Many people would ignore a spot like this because it's separated from the bulk of the layout by the tracks. I put down a small dirt road which leads to the edge of the layout suggesting that it starts before you reach the layout itself.
If your layout has streets then adding a little detail can make a big difference. Draw inspiration from the streets you drive on each day. Streets have patch marks, they have sewer drains, and most of all, they have lines. Getting perfect lines on a curvy street is near impossible in Z scale, but in larger scales they're much easier to do with decals, stencils, and patterns you could follow. When I did my storm drains I even removed the part of the layout they covered and painted it black so when you look at it you don't see it simply placed on a painted street.
Weeds, shrubs, flowers, and tall grass are easy to add to any layout and help add some color (particularly flowers).
Tall grass in a water drainage way are easily seen against the dirt.
Use figures! Have them doing every day things like going to the Post Office or going for a jog. While figures waiting at a train station makes sense and looks good, having figures in motion adds the sense of movement to the layout.