When trying to photograph a city, one basic step to avoid missing good photographic opportunities is to have your camera out and ready. Camera bags and lens caps are useful for transportation between cities, but they are not things that you should have between you and the subjects you are hoping to photograph.
In a similar vein, it is wise (and good practice) to adjust your camera settings whenever there is a major change in the light around you. When you leave your hotel, for example, you might want to switch to a low ISO setting like 200 or 100 because it is bright outside. Along with that, some suitable settings might be a medium aperture like f/8 or f/5.6, with shutter speeds set automatically via an aperture priority mode.
If you then move from the bright day outside into somewhere indoors and dark, you probably want to open up your lens to f/4 or f/2.5 (or even f/1.8 or lower if it is really dark) and bump the ISO to a level that provides acceptable shutter speeds.
Changing your settings whenever the light changes accomplishes two useful things. In the short term, it sets you up to immediately and effectively photography anything you see. In the longer term, it builds familiarity with your equipment and with photographic settings. Once you have that, you can change settings on the fly more easily when truly unexpected circumstances suddenly arise.