Bottomless vales and boundless floods, (line 9)
It's pretty clear that the world of Dream-Land looks nothing like any place we've ever been. Poe lets us know that we're in a really different world by tacking on words like "boundless" and "bottomless." These words are kind of descriptive – we can imagine what he's talking about, but they are also a little mysterious. We can't know exactly how deep or wide these oceans and valleys are. Poe wants to keep our vision a little hazy, a bit dream-like.
Mountains toppling evermore Into seas without a shore; (line 13-14)
Crazy stuff is happening in this landscape. Not only is everything really huge, but it's churning and exploding and collapsing all the time. In the real world, a mountain falling into the ocean would be a pretty big deal, but here it seems to happen all the time. This explosive action adds a little bit of excitement to the poem, makes us feel as if incredible things are happening – as if we were watching a movie with great special effects.
With the snows of the lolling lily. (line 20)
This is sort of a funny, quiet little image. It's not scary or overwhelming or gloomy like a lot of the other natural stuff in this poem. We imagine a pretty white field of snow. As for those lolling lilies, maybe Poe is using the image of a white lily to echo the whiteness of the snow, or maybe there are actually little flowers poking up. Either way, this is kind of a relaxing, happy natural image in a poem that is otherwise pretty intense.