Donald's Boat (officially named the "Miss Daisy"), doesn't go anywhere, but there are still a lot of things to do for kids of all ages. Like most of the things in Toontown, this is a "hands-on" place, and kids can enjoy blowing whistles, ringing bells, and lots of other fun stuff.
One reason for adults to climb up to the upper deck is that you get a panoramic view of central Toontown and Gadget's Go Coaster. Don't try and climb that tiny staircase inside the boat though; it's much easier to go around the back side of the boat, down the walkway that runs behind the waterfalls, then up the normal-person-sized stairway.