I believe that Ryanair has flights to Baden-Baden/Karlsruhe, Memmingen and Friedrichshafen, but I don't know where they depart from. You can reach Stuttgart from any of these places using a Baden-Wuerttemberg day train ticket for 22€ for the first passenger and 4€ for each additional passenger up to a grand total of 5 people on a ticket. This ticket is valid from 9AM to 3AM the next day on Mondays through Fridays, and from midnight to 3AM the next day on Saturdays, Sundays and holidays. No fast long distance trains can be used (IC,EC,ICE). If using this travel option, check "use only local transportation" on the www.bahn.com website and only valid trains will be shown. Otherwise, any means of valid transportation can be used within this German state (buses, trams, U-bahns, S-bahns, regional trains). Because Memmingen is in Bavaria, only the regional trains along lines leading directly to Baden-Wuerttemberg (e.g. via Ulm) are covered by this ticket, not the bus from the airport to the train station. Note that the B-W ticket option will not be given as a travel option from Memmingen to Stuttgart as there is a glitch in the train software which ignores valid journeys beginning in a state outside of Baden-Wuerttemberg (also true for other German states' tickets). However, the software works fine going in the other direction.