This is rather difficult to decide.
No single lens can cut it in situation you've given, except maybe 18-200mm f2.8 which doesn't exist yet.
With only one lens, you'll be limited to how well you can shoot. What sort of photo opportunity would you be prepared to give up in favour of others in that case?
Even with crop, neither the kit lens or 50mm can get you the detail the Sigma could otherwise give you if the subject is far away.
While the 50mm is better at low light, it does not offer the versatility of the zooms. Can be a pain in some case.
With the Sigma, night shooting of the rides and cars on display would be more difficult. I pressume you won't be happy with the result.
Last, the kit lens is slow and short.
That all been said, I figure you'll be happier with wide angle. Bring the kit lens.. You still can get shots of some birds if you go up close. If possible, get a gorillapod. It is small enough to fit in your pocket (http://cameralabs.com/reviews/Joby_Gorillapod_SLR_Zoom/
) and useful when shooting the ride at night.