This is going to be pretty short and not that helpful since I did Coleridge last term and haven't had looked over it since:
Kubla Khan is an imaginative journey because it takes the narrator (Coleridge) and the reader through an imaginative journey. In the first stanza the rhythm is pretty much constant and the imagery used is fairly safe and restrained (I think some quotes are 'bounded', 'limited', etc but not sure). Then in the 2nd stanza it becomes wilder, the flying fragments, demons, etc. Then the last stanza becomes restrained again. The narrator is drawing you into the imaginative journey, first through the order, then through the chaos, then returning to order.
With Frost at Midnight... I'd have to get back to you on that one. Err I'm just thinking that what I said on Kubla Khan made absolutely no sense :S