You may exit ANY attraction at WDW after walking through the queue,
without boarding and riding the attraction.
This goes for everything from Winnie the Pooh and Dumbo up to Space Mountain, Tower of Terror,
and Expedition Everest.
Just tell the "loading CM" that you decided not to ride.
Its no big deal at all.
The more involving/interesting queues at WDW are:
Pirates of the Caribbean
Tower of Terror
Rock'nRollercoaster
Expedition Everest
Its Tough to Be a Bug
Dinosaur
Test Track
And the PRE-shows to the following can be fun by themselves:
MuppetVision 3D
American Adventure ("Voices of Liberty")
Ellen's Energy Adventure
Special note:
In Haunted Mansion, there is even a "Opt-Out" Exit, just after you leave the Streching Room.
On your right, you'll see a door with this sign. That's the exit. (It also doubles as the wheelchair entrance.)