Go with the flow! Sometimes plans change unexpectedly. Your tour gets cancelled, or the place you wanted to eat at is closed, or the weather isn’t right for a hike. When this happens, you have two choices: have your day ruined by something out of your control or make the day into a spontaneous adventure.
Now, I should note that safety is most important, so by no means should you enter any place you might think is unsafe. So if you’re unsure, stay on the side of caution and be safe! With that said, use this as a chance to explore. Maybe you’ll stumble across something great, like I did in Ostia Antica!
Your approach and attitude can make a big difference, too. When I visited Lisbon, I was so excited to wander around the beautiful city and the ruins of the castle. But, when I woke up on my first day there, it was raining! I decided not to let the rain spoil my adventure and wandered around the ruins anyways (umbrella in hand). It was fantastic and the view of the city during a storm was breath taking.
So, don’t think about what you wanted to be doing, think about what you are doing. And if you miss out on the chance to do something you really wanted, hey, that’s just another reason to visit again.