I know that you booked a rental car already, but to keep things straight, there is no need to take a taxi to Rosenheim when wanting to go somewhere from Aibling, as Bad Aibling is served by trains to Rosenheim and other cities at least once every hour (especially, if you want to visit big cities like Munich or Salzburg, where driving/parking is not that much fun). Every German knows about this train line because there was a head-on-collision with some casualties about a month ago.