This is all about stopping that flat look in your park. To be honest, if your park is very dense and full, it doesn't matter too much if it's flat. Interesting scenery and theming can make a world of difference.
Part 1 - Heights:
If you want things to stand out a little, it's well worth considering the height factor. Before you start, you need to play with all the terrain editing tools and learn how to use them. I'm still working it all out, but find out how to get rid of jaggy edges using snap tools or smoothing. It's worth spending an hour or so just making hills and valleys to learn the system.
You can see here that Guideland Park is already looking nice an hilly. There aren't many attractions though. All grass and no rides makes Guideland Park a dull boy. In a future guide, we'll see a nice centre piece on top of the hill in the foreground, but in the background, there's the perfect place for a bit of ride magic to happen.
So we'll have a mine drop ride and top spin. Not a great choice, but it's all we have. Firstly, a mine would be somewhere where there's something worth mining, like on a rocky hill. So using the terrain raising tools for hills, we raise the land into a very big hill. I used a 20 or so sized "brush" for this. It helps keep the gradient smooth, but raises a lot of land. Then I used some smaller "brushes" to alter the land around it to give it a "Cliffy" feel. You should recognise all of this from the previous guide. I've sunken the Top Spin into the hill wall. I had to flatten a large area of land before putting it in:

Before moving on, I've got rid of all the block edges around the path area using the "snap to neighbouring edge" tool. I've used the same too to dip down to the Top Spin area too. You'll notice that I had to move the entrance too (and I added a themed one while I was at it). Sometimes you make a mistake, but if you've given yourself lots of room, you'll be fine. I've left the sheer face at the back of the Top Spin, as I couldn't decide if it would look good or not.:
We now have a mine drop on top of a steep hill, and the Top-spin is tucked away at the bottom. It's a bit excessive for a mine drop ride, but it looks good, and you could maybe run a coaster by it...
Time to add the finishing touches. I've used a rock terrain paint to add some decent texture to the hills, and then put a fence running right down the pathway.
A few bits of odd scenery here and there and a "shed" and things are starting to look better.
Just for completeness, I've removed the sheer face behind the Top Spin. It looks a lot better "smoothed out". I've extended the path to its final destination, and played a little with some more terrain paints to touch things up a little.
You'll notice that the theming on the ride stations isn't as extreme as it was in the flat ride guide. For this Western theme, and with all the differences in height, you don't need to go as mad with the theming. The landscape itself is enough. Large station buildings and the like would be overkill. In this case, less is more:
As you can see, the area isn't really any bigger than the same flat area with these two rides would normally be; but it looks a damn sight better than a basic flat layout. You may lose some flexibility when it comes to adding attractions, but if you use your imagination, those limitations can just help make things even more interesting.
Part 2 - Hills:
Sometimes you don't want to build a coaster up on a hill, but you do want the landscape to look less flat to avoid giving it that "car-park" feel.

The obvious solution is to dump in some hills:
Oh dear, you're theme park appears to have a case of pimples. Yes, they're hills; yes, the game will treat them as landscape close to the coaster and increase it's rating a little; yes, it's rubbish.
Hills don't just sprout out of nowhere conveniently around your coaster. Hills are a natural(ish) occurence and "undulate" nicely. So once again chose a large "brush" and put lots of raised terrain under your coaster.
I've had to use the terrain-smoothing tool to get rid of some ridges that you get when getting close to the coaster structure. As you can see though, it's hilly, but not pimply.
You can still add great hairy lumps if you like (for whatever reason). Just remember that grass doesn't grow on sheer faces:
It doesn't look good to be honest, but if maybe you were creating Death Valley or something?
Anyway, there we are, "hills and heights": two ways to try and bring your park out of the doldrums.