Haha, I'm digging Marvin the Martian's and Space Ghost's faces. As far as balloons, the only thing that might be problematic, is that, depending on how many you put in there, it could be too busy. It you put a few, or maybe a small cluster or two, it could be effective. Also, if you wanted to give more of a party vibe with the background, something like a banner could work too, and not be too busy.
Thanks!! A banner is a really good idea!! i might be too lazy to mess with it... but if I do, I'll give that a try.