Are you going to rent a car? A question I hear quite a bit when talking about traveling, and the answer to that question is almost always no. Like many people in the US, until a few years ago, I had never used public transit. Much of the country is made of minor cities with at most a bus transit system, but even that was something I never used when I was young, my hometown is too small and I grew up with cars as the only option.
Roughly 4 years ago I traveled to Washington DC and was forced to use the public transit system, I had no idea what I was doing, and wasn't sure I was going to get it right. Somehow, I managed to get off at the correct stop, then it was so disorienting to exit the metro for the first time, get myself in the right direction and to the correct hotel in downtown, at night, where I hadn't been in years... well, everything changed after that.
I had become a huge proponent of using transit system. It's so very easy to get around, the lack of stress about other drivers, it's just easy. Since that date, this has become the preferred method of travel in a city. As one who travel quite a bit now, and I don't buy many souvenirs on my trips, I have taken to collecting transit cards from every city I visit. This is my little version of a souvenir... and it is practical because I can go back and use them again when i make work trips or decide to go back.
Most major cities have a transit system. For my trip to Seoul in a month, I have already planned out how to get around on the transit system. My Google Drive has a transit map already uploaded, and I haven't planned every trip since I don't plan that detailed of a itinerary prior to arrival. For the major location that I plan to go to, those I do plan out in advance since they tend to be a more complicated route.
Just keep in mind that you may want to get some idea of how to get a transit card, personally I prefer the tap on and off cards since they are so much easier to use and make for a great memory of my trip, but many systems allow for single use tickets or a pass that covers multiple days. Paris for instance didn't have the cards I like to use, so I purchased a multi use pass and have used that for my travel and for my souvenir.
There are also other options, like Uber, local taxis, buses, and long distance trains. Working in France for a week last year went like this. Fly from Philadelphia to Paris, catch the TGV high speed train to Aix En Provence, bus to the city, and a daily bus to work. It's a bit of a mess, but for the back and forth, not a huge deal. Next time I'll skip the train and just fly to Marseille instead. That shrinks it to 2 flights and a bus ride to Aix. Simplification is the name of the game.
Also keep in mind that Uber offers a promotion where you can earn Starwood points (SPG) for every ride after your first stay with a Starwood hotel. If you ride Uber while you are staying at an SPG hotel, you get even more points. take advantage of that is you need an occasional ride with Uber, or if you use it all the time.
Transit isn't right for every situation, long trips to remote places still require a tour bus or a car, but for the kind of travel I do, transit works perfectly and I think it's better for keeping down the stress that tends to come from driving.