You may see Cathedral of the Holy Trinity in the skyline before you actually find this important building. Step inside if the doors are open or just take in the facade from the street.
You'll be spoiled for sightseeing choice when in Old Quebec. Cathedral of the Holy Trinity is an ideal place to kick off here, but there are numerous other worthy places to check out. Place Royale, Laval University, and Montmorency Falls are near the principal church and will round out your day nicely.