Afternoon on Montmartre. One of the more picturesque and walkable part sod a picturesque and walkable city (Paris), Montmartre is among the most fun and bohemian neighborhoods to simply lose yourself. Dozens of cafes, fun shops, and terrific people-watching in an area that is ever-so-slightly Paris' most important artists' enclave, giving rise to the Belle Epoque era.
Capping the hill is the magnificent Basilique du Sacre-Couer,
with perhaps the best "on-high" view of modern Paris in the city.
Wander the streets, grab a bite of lunch, then take the funicular up to the Basilica to gaze out over the city as you watch the people go by. It's a guaranteed great day in Paris.