As far as breaking up the monotony of the trees, just make two or three silhouettes then tile them in a random order.
As far as keeping them less defined, to make sure they don't look playable, it's more a question of contrast than definition (but that definitely plays a role) So say you've got a four colour pallet, make background objects two shades that are right next to each other, foreground objects (tiles) three shades (they don't necessarily have to be next to each other)
then interactive elements the whole range, or at least the lighter colours. Obviously this is just an example and not a set in stone rule, but I hope you get what I mean.
You came here looking for help on the mech though, have you got any attempts (even failed ones) that you can show us? Hopefully if I get the motivation today I'll give it a shot to show you what I'm talking about with the contrast.