Well, a good start would be first, watching TMP, and learning what you can from that of the refit... also, Mr. Scott's Guide to the Enterprise is a great resource on the refit Enterprise.
As for what time travel element would be interesting, hehe... that is something YOU as the principal author must determine... you MUST, and I cannot emphasize this enough... you MUST have the basic plot outline for the entire story down pat, before you even begin to write anything.
That's it, for starters.
I have had the stories firmly set for ALL of my own fics:
-Star Trek: Entomalians
-Star Trek: A New Beginning
-Star Trek: United We Stand
-Star Trek: Distant Star - "Waking Nightmares"
-Shades of Gray