With time periods, there is a distinct difference. "In a few days" means "(almost exactly) a few days from now". Not tonight, not tomorrow, but roughly a few days from now. "Within a few days" means "any time from now until a few days from now". Maybe tonight, maybe tomorrow, maybe two days from now, but NOT more than a few days from now. With locations, the difference is not that important. But, I think there is a slight difference. "In an X" can refer to a specific spot and can also refer to a stationary activity. "Within an X" is more vague, there is no specific spot and it can refer to a movement-related activity. For example: I left my keys in the house. The dogs were running around within the house. Still, I would say "The dogs were running around in the house."