The Altima is quite light for the size and it has fairly good aero already. A grille block (partial or full depending on season and location) and max sidewall tire pressures should get you going.
Is it MT or AT? 4 cyl. or 6 cyl.? Sealing panel gaps could be useful, but you have a great starting point already. Unless you go with extreme mods and add a boat tail, I'd stick to non-intrusive mods for now.
My mother in law got 40 mpg out of her 4 cyl. AT Camry driving to Oregon (from WA state) by just setting the cruise at 60 mph for the whole way. Just letting you know what's possible without any mods. Low hanging fruit!
