I came across
these in my browsing. Aurora looks the most interesting to me, has a nice way of adding skies and water effects. Verdant looks fascinating, to add photoreal plants, but the individual plants are very expensive (though infinitely reusable). Modelshop could also be fun, lots of free models to import on the internet. Not cheap, but they have free demo downloads. Aurora might be worth it, and has "movie" tutorials on the site.