So my buddy Utack just let me know something pretty neat, and while some of you may already know this, it's pretty valuable information to those that don't.
You can acquire both the Acro and Mach Bikes from Rydel if you talk to 3 specific NPCs. A Hex Maniac in the Desert North of Mauville (requires Mach Bike), some guy on the rainy route West of Fortree (Requires Surf, Waterfall, and Acro Bike), and a biker at the Battle Resort. Once done, talking to Rydel will get you whichever bike you don't currently own. This is also the only way to fully explore the Safari Zone, so that's neat.