Central Passau is sufficiently compact to explore on foot. The CityBus links the Bahnhof with the Altstadt (€1) up to four times an hour. Longer trips within Passau cost €2; a day pass costs €4.50.
The walk up the hill to the Veste Oberhaus or the DJH Hostel, via Luitpoldbrücke and Ludwigsteig path, takes about 30 minutes. From April to October, a shuttle bus operates every 30 minutes from Rathausplatz (€2).
There are several public car parks near the train station, but only one in the Altstadt at Römerplatz.