The Las Ranillas Hydrothermal Center is a great choice if you're looking to relax a bit during your trip to Zaragoza. It's a sprawling fitness center and hydro-thermal spa center in the Expo area of Zaragoza just across the futuristic Third Millennium Bridge from the main bus and train station.
I'm assuming you won't be using the full fitness center on your vacation, but the spa is more than recommendable. You can visit the hydrothermal center for 2.5 hour blocks (more than enough) and enjoy the Swedish saunas and mist showers, then sweating it out in the Turkish bath and taking a bracing plunge into the cold-water pool. Then you're ready to head to the large warm-water pool that's filled with different stations that will bubble and massage your sore body to jelly-like bliss.
It's open until 9pm Mon.-Sat., but remember to bring a diving cap and flip-fops, both mandatory to enjoy the spa. You can pick them up for a few euros at any local Chinese general store. Enjoy!