Lac Daumesnil is an excellent place to get away from the bustle of daily life. Check if any water activities are available here, or just soak up the serene ambiance.
You'll have no end of sightseeing choices in 12th Arrondissement. Lac Daumesnil is a good place to start here, but there are loads of other fine sights to visit. Eiffel Tower, Louvre Museum, and Notre-Dame are not far from the body of water and will fill in your day nicely.