What is the ruling on a niqab-wearing or hijab-wearing woman riding a bicycle or a scooter, and is it permissible for her to practice on skateboards in uncrowded times and streets?

It is permissible for a woman to ride a bicycle and the like. The basic principle is permissibility unless it leads to a forbidden consequence. If strange men see her, then it is forbidden, due to the difficulty for a woman to maintain the required covering, as her clothes often describe her charms, and some of her body might be exposed. Indeed, women were prevented from trotting (ramal) in the tawaf and sa'i because it goes against complete covering. However, if there is a necessity or a strong need to ride a bicycle on a path used by men, because the harm is less than riding mixed transportation, then the necessity is estimated according to its extent.