This will not officially be decided till September 2013. At the moment there are three cities who have been proposed to host the Olympics after next: Tokyo, Madrid and Istanbul. Other cities which were rejected or finally decided not to bid include Doha in Qatar and Baku in Azerbaijan, as well as Rome. Rio de Janeiro has already been chosen for the 2016 Games.
Tokyo has hosted the Olympics before, in 1964. It has also staged the Winter Olympics twice as well as international football and other sporting events, so it clearly has the capacity. Earlier Hiroshima was considered as a candidate city, but this idea was abandoned.
Istanbul has also hosted a number of internationa events, especially football, though the Olympics haven't been held in a Turkish city yet.
Spain has hosted the Olympics before, but that was Barcelona in 1992. However, Spain also has a history of putting on major events.