Oh, but that's no fun! Unecessary linguistic complications make the world go round! Besides, there's no *true* synonym in English--they all have different connotations (yeah, I'll just tell myself that...).
As for sounds . . . you could check out some Asian languages, as you didn't list any and it seems likely they would have some (Japanese "r" comes to mind, though I'm not sure if it's quite what you're looking for. It's pronounced as sort of a combination r/l that ends up sounding similar to--but not exactly like--"d." Heh heh. Well, to me it does. But I don't know if having that kind of r-sound would be a good idea in a language with two other ones already, especially if you're aiming for unconfusing and easy to speak.)
No poetry, no puns, no metaphors. . . . Hey, you could always learn a computer programming language! ^_^ (Actually, that might be an interesting thing to look at--computer languages. Give you ideas and whatnot. Or math/music.)