Yes you can make a reservation even though you are not staying at the Fort. You won't need it for Trails End since its so close to the boat dock. But you can drive it and park right next to the campfire, also looping is popular to look at all the decorations at the campsites. Its best to make a reservation well before the holidays because they will book all their golf carts.
Advance reservations are highly recommended and can be made up to one year prior to your visit by calling (407) 824-2742. Unreserved carts may be available for rental on a first-come, first-served basis.