Ok, really it depends on shot to shot. You need to decide what your shot is going to be. Then you can go about getting it. Ok for a really close up shot. Say like blizzard.. diablo.. the crow and grass.. Ok,, then i would suggest either enomorphs. Or bones and dynamics. You can try it with one grass,, then clone it,,, using various techniques.
Ok for a quick grass,, you can try saslite. The coming in either x y or z will help you get a messy wind blowing sort of look,, if the shot is a far one.
The same goes for trees. For a close up shot, a lot of attention to detail is needed so you really need to think it up, say a storyboard of how the shot is going to be done helps loads.. even if its only 1 sec animation.
Oh ya check out the tutorial here about grass and trees,, and a little animation.
http://www.simplylightwave.com/movie...tml?tut_id=505