I can make some good songs on guitar, but my band can't quite figure out any good parts for their instruments that aren't the most basic parts possible. How can we write more exciting peices that still work together?
tell them to work harder at it, or give them an Idea of what you want them to play.
"Swords, nature's hell sticks."- Trip Fisk
Master a single key, like Gmajor, and know all the fret positions in that key. from there, learn as many songs in possible in that key, then when you write your own song, u just have to transpose all the notes + tonic up by what ever interval.

good luck!
before someone shoots down this thread because "itz in da rong forumzorz!!!" i would say that this is a problem that will naturally work its way out through years of hard work. you need to start very simple and unexciting, and you will be able to branch out into more interesting stuff
yeah, looking back, the songs i wrote for my first band were WEAK compared to what i write now. and the parts we would come up with to fill them out were basic too... just keep plugging away, and look into some music theory, it's always helpful.