I've arranged to have my organs donated upon my death but otherwise I haven't really thought about it. I don't think there's much use for a body or a coffin after death, what's the point? It's not like you're going to use it. But I wouldn't want my ashes to be in a jar on someone's mantel either.
I guess I'd want to be creamated, have my ashes scattered somewhere sentimental and then have a tree planted in my name in a park somewhere. So if any of my loved ones I leave behind want to pay their respects, they would have somewhere to go to remember me.