Go for a walk by the water's edge and soak in the views at Apra Harbor. The blue sea and bobbing boats make for a great photo opportunity.
This place for ships to anchor in Piti is well-liked by travelers, so make sure it features on your travel itinerary. Don't think that's all there is to do here, though. There are many more attractions waiting for you to check out after Apra Harbor. Guam Museum, Gab Gab Beach, and Statue of Liberty are nearby.