I'm very pleased you and your friends are coming to Zaragoza.
There are several sites that you should visit, but if you are looking for not usually spots, I'll recommend you near Zaragoza, the Bardeas Reales, an amazing nature space where you could see very strange and artistic landscapes.
Nearer Zaragoza, you can visit the Academia General Militar, where you can discover how the army organizes the troops. It's one of the biggest headquarters of Spain.
In the meantime, I recommend you to go through the Tubo, where big amount of bars full the street. Local people usually have there the aperitif and some tapas like "la ballenita", a piece of bread with herring on it. How tasty it is!.
Keep me informed
VeN