From Mauville's end of the Cycling Road, head straight to the right along the narrow path. At the end, you'll find a glowing patch of ground in front of a trainer. Defeat the trainer first and collect the Mega Stone Manectite.
Beauty Melissa (Reward: p952)
Continue along the Cycling Road to face the following trainers. They are all using Electric-type Pokemon.
Triathelete Sloan (Reward: p680)
Triathelete Jacob (Reward: p640)
Triathelete Sloan (Reward: p680)
Triathelete Benjamin (Reward: p680)
Triathelete Dolph (Reward: p680)
Also, if you entered the road from the north, you'll be evaluated by a cyclist in the southern end of the road depending on how fast you reach the end. Achieving the best time doesn't necessarily give you anything but it's something you can aim for if you want.