The Ile Saint Louis is the perfect place to have a romantic walk ( it's less crowded than the City Island, because there aren't  main monuments). You can enjoy some delicious ice cream at Bertillon shop : it's supposed to be the best ice cream in Paris, but be carefull the shop is closed during August. 

 

In the main street, "Rue Saint Louis en L'Ile",  there is a fine gallery (L'ile aux Images) where you can find lots of original photographs and lithographs. Probably the best photo/posters/lithography gallery in the area. It is really worth a visit.