Just thought I'd share this thing I do sometimes to kick-start the creative juices... (though I'm not actually using it that much for Falling Water, as that level is supposed to have a rather 'real' look about it, for the most part, and this technique works best for levels that are a little on the surreal side).
1.) Build a screen using some standard tileset (better yet, two tilesets).
2.) Keep replacing the tileset/one of the tilesets simply by stepping through the tilesets in the editor; don't actually change the composition of the screen. See what serendipity comes up with for you.
90% of the time this'll produce random crap; ten or five percent of the time, though, it will produce... seeds of brilliance that, with a little bit of effort, can be turned into really interesting screen designs. See below for some examples of such 'seeds'.
None of these are actual screen designs yet; but I can see the concept of an interestingly surreal building/structure in numbers 3 and 4, and the potential for interesting 'landscapes' in 1 and 2... 1, especially, could probably make an interesting look for a nice little challenge level. I love the contrast between the black-and-white abstractness and the three-dimensional, bright-green vine. I could imagine a level with a white abstract background, black abstract ground, and floating vine 'blocks' to jump on etc. (Ignore the thatched hut; that would *not* be part of the challenge level's design.)