Unusual yet very positive feature of Paris - a river bank redeveloped into a stretch of cycleways and pathways. When the weather is nice, take a stroll along the river, admire the historical monuments or play some game. The pathways are great for many free time activities (often there are some organised) - plus the loungers are especially inviting!
Wikipedia (French) http://fr.wikipedia.org/wiki/Promenade_Berges_de_Seine
Address Quai d'Orsay, 75000 Paris, France
Coordinates 48°51'47.027" N 2°18'37.314" E