My mom likes to work in the garden. When I was a kid and still living at home, I would often help her out. She'd have me do mostly fun stuff, like chopping things up with an axe or doing lots of digging (yeah, that was fun when I was
young). Occasionally she'd let me tag along with her to the nursery, where she would always let me buy a plant of some kind to put in the garden. It was when we were planting whatever we'd bought at the nursery that day that I learned the Rule of Threes.
I cant remember her words, but the Rule of Threes basically states that things are aesthetically pleasing when they are set up in threes. A little cluster of three flowers is, for some unknown reason, more pleasing than a little cluster of four.
Now obviously this isnt true of all things, but I think its a pretty good rule of thumb for D&D design. If I'm going to give the PCs a crossroads, more often than not I will give them three choices. For example, yesterday I was coming up with a way to incorporate a new monster I'm designing (which I'll post when I finish the stats). I knew I wanted this monster to appear in a valley.
What I did eventually was make an area the PCs had to get through to reach a major objective. On either side of this area are huge impassable mountains. In between the mountains are two valleys, separated by a ridge that runs down the middle (these all run parallel to the impassable mountains). To get through this region, the PCs can choose to travel down either of the valleys, or traverse the mountainous ridge in the center.
Here's where things get interesting. The ridge in the center is infested with rocs and wyverns (this also helps me dictate just how much the paladin can fly around on his hippogriff, since the inhabitants of the ridge are pretty hungry). The eastern valley is full of the new monsters I created, called ulalumes. The western valley is full of these parasitic creatures called urges which attach themselves to your brain stem and make you want to do things like kill your friends.
Now I've given the PCs three different ways to get through the region, and something about it is just right. Not too many choices, and not too few. Juuust right. At the edges of the valleys, I've imagined huge stone monoliths stand, encrusted with jungle vines, which demarcate the beginnings of the dangerous valleys. As for the ridge, hopefully the PCs will notice the huge shapes flying about the craggy peaks before they decide its just an easy climb.