Ok so I'm currently hatching for shiny eevee eggs and I need a good place to ride my bike around so I can pass the time.
There are two very good ones: one in New Mauville and one in the Battle Resort.
They're shown in more detail here: http://gaming.stackexchange.com/questions/193183/is-there-an-infinite-cycling-loop-in-oras-similar-to-the-lumiose-city-tower-i
Yes there is at the Battle resort there is a perfect square where you can go to.
go down to the beach, go in front of the day care boy, go right until you hit a rock and just keep holding up.