yes, save time to spend a day there... there are also evening events. The one day we did stay there, we practically had the place to ourselves, and they have a lovely pool
You can't plan your disney days out with too much detail .... they have activities that may bump into your plans... be prepared for your park plans to change for a GKTW activity. Some are easy to miss, some are so special you don't want to miss them.
I very much regret that i had set up a Pirate's Adventure for my two DDs, (DS, DH and I were going to ride the monorail and go into some resorts, just bum around)... well that was a day that GKTW had a special surprise... I didn't think clearly enough to call Disney and say - i can't bring my kids to Pirates because.... so we missed out on a very special moment. SO DUMB I could kick myself...
So, again, don't be caught up in going TOO Disney.... especially since you have time AFTER to do that. Having said all that though, the Disney "treat this kid extra nice" pass may expire the day you check out of GKTW, so bear that in mind. I'm not sure that will happen with Universal.
Take some time to sit with the nice volunteers who will tell you all about Disney. They can guide you to some special character interaction opportunities etc. You will be surprised at how fast you can get to rides, etc, so other than mapping out a route and picking rides, you don't need to worry too much about fast passes, etc.
I am glad you are staying on for longer than the Wish Trip... you go to a lot of effort to get there, take your time! We stayed extra - and of course while GKTW did not pay for the extra time with the car rental - somehow we were given the extra days at their discounted price which was very nice.