Since the Outdoors Challenge is one I particularly wanted to see in the challenge list this year, I figured I should do the inspiration post as well.
The Challenge is specifically
The Great Outdoors - Get out into the weather and dirt with an item for outdoor pursuits. When I began searching for examples, I found a slight issue...there really isn't outdoors specific clothing for pre-1600s or so. Other than outermost garments for protection from the elements, it was the same as any other clothing--likely because people did tend to spend more time outdoors.
What that means, is that it should be quite easy to find something which qualifies; it could be for an activity specific outfit or garment as for riding/hunting, swimming, playing in the snow, walking, or cycling. Alternatively, you could go the other, much more generalized route by making something simple which is meant to get dirty (or again, keep you warm. I'm in the middle of winter here, and somewhat focused on warmth....) while you work outside.