At Mt Vernon Park, there is a wide grassy area surrounded by bushes and trees that resembles a 'Glade'. It is an easily accessible spot with parking nearby, meaning it is only a short walk. This area is extremely muggle-populated during the day so caution and sneakiness is required.
You are looking for a small transparent jar that I believe comes under the micro category. It is sitting in a tree between the two flax bushes (there are multiple trees, so you will have to check all of them!). BYOP!