Well, you can use any model and use it as terrain structure like cliffs, rocks, plants, or whatever. The model can be quite detailed both in structure and its texture. Make a big model and decrease its size when you assemble the level to increase the appearance of detail. Same goes for using hmp terrain files. You can spread out a few small terrains with good textures on top of the main terrain, decrease their sizes to get really high detail much the same as you do models above. There is nothing that limits you to use only one terrain entity. Read about the differences of hmp and mdl files in the manual. Just make them passable if necessary. Only imagination is your limit.