The Historical Museum of the Institut Fondamental d'Afrique Noire traces Senegal's history from prehistoric times through the era of European contact and the slave trade, with exhibits on traditional culture, colonial history, and the island itself. It is a good complement to the Maison des Esclaves, providing broader context for what you've seen.
The collection is modest by European standards but informative, and the building — an eighteenth-century fort — is worth seeing in its own right.