The old city is very walkable. Other options for getting around are:
By bike: https://www.bizizaragoza.com/en/info/subscriptions-and-prices
By tram: https://www.tranviasdezaragoza.es/en/
By bus and train: http://www.consorciozaragoza.es/
Taxi: +34 976 751414 or +34 676 207878