For lava, try using the volumetric fire material and make it standard, not volumetric. Applying this to a "Cauliflower Hills" terrain should work pretty well. For steam, apply volumetric cloud materials to random size and dispersion spheres. Watch the overlaps, as they will interact with one another. Most of what you are wanting to do can be done in bryce, but it will take a little work to get to look right. The effort is always worth it though. Whether or not the picture turns out the way you want it to, you will learn something that will make the next picture that much better.