First the single most important thing, CONGRATULATIONS!!!!!
Now, to the rest of your post.
WDW is enormous. One day at MK is like having a single leaf of lettuce as an appetizer for a starving person. Well, it is! LOL.
So, what to do there in one day? What do you and your soon to be spouse like most? Thrill rides? Shows? Parades? Character interaction? Where do you plan on eating the day you are at MK? Do you have any desire to see any other part of WDW?
You'll need to decide how you want to spend your time, and then fill in those hours with the activities that are most appealing to you.
My wife and I are not thrill ride people, so a day at MK goes something like this for us...
Train ride around the perimeter (at the entry) just to get the MK vibe going. Splash Mountain, Big Thunder Mountain Railroad, Pirates of the Caribbean, It's a Small World (yep, we love it!) and whatever other "small ride" we may want to do. My wife won't do Splash Mountain with me.
We like Mickey's Philharmagic, Wishes and the Main Street Electrical Parade (a night parade, that will leave for California at some point in the future).
We don't often go to MK unless it is the first weekend in June

or if we have young relatives/friends who really want to go there. Too many strollers, and there is no alcohol served in this park.
Yep, call us fussy be we like a glass of wine with our dinners.
So, if that helps a tiny bit, great! I'm sure there are others who will come by who are much more versed in the joys of MK.
If you want to take a break (and have a drink

) you can always take the resort monorail from MK to one (or all) of the resorts that border the lake. The all have bars or other places to have a drink and relax away from the chaos that is MK.