If you are young enough to stay in youth hostels, then you probably already know about free walking tours in major cities. If you are not in the age group to stay in hostels, then I will let you in on a secret.
There are many companies that provide tours in over 90+ countries all over the world. The convenient part is that there is no pre-registration required. You simply show up at a specified location and begin your tour. The guides are local professionals who know the area and are passionate about delivering a great experience. Tours usually begin at 11am- no matter what city you are in. The catch is that the tour guides work for tips. If you have a great experience, then the guide will get compensated.
I recently enjoyed a tour in Ljubljana, Slovenia with a guide named Tina. She was enthusiastic and professional. There were about 15 people on our tour. Unfortunately, the freezing rain and cold temperatures were too much and after 1 hour, our foursome tipped the guide and bugged out.
My second experience was in Zagreb, Croatia. At 11am we met in the main square. This tour had about 20 people. I think we gathered more as the tour went on. Our guide, Luca, was knowledgeable and funny. We toured for 2 hours and had a very good experience despite extremely cold weather. At one point we had snow flurries!
I love the flexibility of theses tours and not having to make a reservation. I highly recommend checking out these tours on your next trip.