True MVP: Free Walking Tours
When looking into things to do whenever you visit a city, a common activity is a "free walking tour." Seems like a pretty sick deal, and possibly too good to be true? Free walking tours are typically incredible, with local guides who are knowledgable about nearby food, history, and customs. Often they'll teach you a little about the language, architecture, and cool places to spend more time after the tour.
I ALWAYS recommend doing a free walking tour the first day you’re anywhere, it orients you in a new city, and it is so much easier to navigate yourself later around the city.
Sandemans is a very popular and very good walking tour company, which I've used in Amsterdam, Prague, Munich, and Barcelona. The tour guides I've had through them were amazing. Additionally, they also have other tours for relatively cheap. I went on a biking tour through Barcelona for only 16 euro. Considering rent in a bike for a couple hours in a city will be about 14 euro, its a really good deal.
If you don't want to use Sandemans, or they're not in the city you're visiting you can TripAdvisor the best in the city.
However, it is very important to understand how "free walking tours" work. Do not be those rude and terrible tourists who actually think the tour is free and don't give any money, despite the tour guides being pretty clear how the whole deal works.
Nothing in life is actually free, and these tours are no different. These tour guides work on tips. All "free walking tour" company guides are freelance guides, and make money exclusively on your tips. The idea is you tip the guide as much as you think the tour was worth. They’re usually 2-3 hours, so I usually give between 10-15 euro for their time and expertise. Keep in mind it is for many hours of work, who have trained, and offer expertise which will be helpful in your future city adventures.
Free walking tours have always been a highlight of my trips in a new city, and always help me figure out what I want to see and do in the city after the tour. Also, often in a city there are sights to "see" but nothing really to do, like a famous building. You'll see these on the tour do you don't have to walk there after. Which is nice.
picture of courtyard in Basilica of Santa Maria Novella