I just use tons and tons of the little round, bottlebrush style K&M trees, they come in a variety of colours and styles/sizes. You need to base them on metal bases so they don't fall over though. They aren't cheap but I think look really good 'en masse' and they are pretty indestructable.
Rivers wise, I use the Hexxon system, but you may think it looks a bit strange if you were putting it down ona normal, 'non-hex' table? (They are on the Kallistra web site if you want to look at them)
Dave