From the hub there is a small path just inside on the right side of the Frontierland gate that goes over to the side path of the castle and leads on into Fantasyland. It has a few benches next to a small pond. I very seldom see anyone when we remember to go through this way.
Also, in the same area from the left side of the castle is a path that goes to Frontierland which cuts through Rancho del Zocalo. Good shortcut if you need to stop off for a potty break!
On the opposite side of the castle there is a cut through that goes by Snow White's wishing well and leads into Fantasyland.
The path from Fantasyland to Frontierland, that 3Minnies1Mickey mentions above, is much less crowded than making your way back though the hub.
There is a small pathway beside Storybookland that is parallel to the main road that leads back to the train station and Toontown which is good to use if the main path is getting ready for the parade.
One of our favorite paths cuts behind the Harbor Galley and runs along the Rivers of America and comes out back by Critter Country. The area has some seating and you can stop to watch the Splash Mountain boats come off the big drop.
If you ever need to get around the crowd on Main Street you can cut through the shops on the Emporium side of town to bypass the people on the sidewalk. You do have to make a short zig zag around the Carnation Cafe but it's easy enough to go right back in the shops and save yourself the hassle of trying to pick your way through the crowd all the way down Main St.