When I was doing the backgrounds for my game I used your games as one of the starting point for developing my own 'quick but still good looking' style. One of the changes I made was sticking shadows on everything - it keeps the cartoony look but adds more depth and interest to the scene. Other than that you could add more stuff in there to try and busy it up.
Anyway, here's the original:

And a quick edit: (paying very little attention to the light source etc)

And here's some shots from my game, if you're interested:
http://onedollarproductions.googlepages.com/Screen1.png
http://onedollarproductions.googlepages.com/Screen2.png
I used Flash for these (as InCreator suggested, I find this sort of thing a lot easier in vectors. You could checkout Inkscape -which is free- if you don't want to download Flash.
You've already got a lot of good advice for characters, I'd suggest you consider how many are going to be in the game, and weigh up how much time it will take to draw them at that level and see if its worth it. You will get quicker though.
Hope some of that is worth considering.
Anyway, here's the original:

And a quick edit: (paying very little attention to the light source etc)

And here's some shots from my game, if you're interested:
http://onedollarproductions.googlepages.com/Screen1.png
http://onedollarproductions.googlepages.com/Screen2.png
I used Flash for these (as InCreator suggested, I find this sort of thing a lot easier in vectors. You could checkout Inkscape -which is free- if you don't want to download Flash.
You've already got a lot of good advice for characters, I'd suggest you consider how many are going to be in the game, and weigh up how much time it will take to draw them at that level and see if its worth it. You will get quicker though.
Hope some of that is worth considering.