Lynn,
You can rent a bike at WL and pedal to the Fort for lunch and afterwards explore the Fort.
I'm familiar with bike rentals at the Fort end but I would think WL works the same way. Rentals when I last paid were $18 a day and you had to return them before the Bike rental location closed.
But in your case it should work with no problem. Rent your bike in the morning, pedal down the wide paved exercise trail to Fort Wilderness that runs parallel to the bus road between the two. Leaving WL you can pick it up out to the left side by the bus pickups to the parks and the original DVC building.
There are bike racks out in front of Trails End. Normally I would recommend a bike lock as well but with a rental you can chance it or bring one from home.
You can certainly bike around the Fort, visit the Meadow Trading Post in the middle of the campground, pedal around the camping loops and then head back down the exercise trail to the WL and return the bike. Also at the Fort you can visit the Horse Barn near Trails End and see the horses that work Main Street in their stalls.
It will make for a great day's activity.
Bama Ed
PS - and IIRC there are no reservations for bike rentals. But if you do it in the morning i can't imagine there being a shortage.