Basílica de Nuestra Señora del Pilar is free entry and overwhelming. I see that La Seo has a separate ticket price, but I swear we walked in free as well. It was Easter Week, so maybe that was it. Anyway, I thought the Basílica was more impressive. Where La Seo shines is the exterior, which is free. As you face the Cathedral, peel off to the left and head down the side. The Mudéjar architecture that is part of the UNESCO World Heritage designation is along that face and quite beautiful. I don't think any of the other museums or sites on the ticket are worth it. In fact the Rosario de Cristal seemed a little cheesy. Here's our trip report from Zaragaoza last year but I'd focus on the firefighter museum and Museum of Zaragoza. As you'll see in the report, I disagree with @ jaimeelsabio and found the Aljafería Palace to be quite interesting, even moving. I loved the layers of history. I just hated the ticket system! I tried to see if the Alma Mater museum had any special exhibits during your visit in April, and came up empty; however, it is just next door to the Basilica, so you can pop your head in and see. Bottom line, we LOVED Zaragoza! You are going to have a wonderful time.