Have a plan.
Seriously. You will waste so much time if you think, “oh, we’ll just wing it!” To be clear: I am all for spontaneity and getting lost. But you will kick yourself when you get home and someone asks “oh my god, did you go here?” and the place they are asking about is relatively well-known like Westminster Abbey or Sacré-Cœur. I’m not saying that you need to schedule the entire trip, but look up what the city has to offer and make a list of the things you really want to see. Then look them up on a map so you have an idea of what is nearby and plan to spend your time in the neighborhood of what you’re wanting to see that day. For me, I’ve had the most success doing this before I leave home. I hate wasting time at my destination on my computer looking up things to do and where they are. Set yourself up for being able to just go and explore.
Here’s my list for what I want to see in Atlanta: