The colonnaded Palais de Longchamp, constructed in the 1860s, was designed in part to disguise a château d'eau (water tower) at the terminus of an aqueduct from the River Durance. The building of this water storage and the associated canals and aqueducts was a major turning point in the history of Marseille as it allowed the city to expand, building new districts. One of these was the Boulevard Longchamp, laid out by the city then developed by private business people who profited from providing a grand boulevard of similarly styled, gracious houses.
In the Palais itself, the 2 wings house Marseille's oldest museum, the Musée des Beaux-Arts and the Musée d'Histoire Naturelle, which have extensive displays of the arts and the sciences respectively. Its lovely gardens with lakes, fountains, waterfalls - not surprisingly water features heavily! - and a children's playground and carousel are a good spot for bored children.
Also take a walk along Boulevard Longchamp and admire the rich architecture.