There is something eerie yet compelling about seeing crypts and vaults. If this sounds interesting to you, take a look at Khanqah & Mausoleum of Sultan Beybars al-Gashankir.
If you're spending a while in Islamic Cairo, Khanqah & Mausoleum of Sultan Beybars al-Gashankir is a fine place to kick off your exploring. This tomb site is one of many attractions that'll keep you occupied here. Egyptian Museum, Khan el-Khalili, and Cairo Tower are also waiting for you close by.