Stuff like, smoke, clouds, fire and stuff can be done by billboarding them. They can all be sprites and then u render them in 2D, and they'll look 3D, the only problem is if I look at something from behind it, I'll still see the same image.

Duke Nukem 3D had like thousands of billboarding stuff. Remember the strippers? when u wanted to look at their asses? but couldn't cause no mater where u went, they were facing you? well that's billboarding..