As I travel around the world I join many FREE city tours. If you enjoy the experience, you tip the guide.
One such company, for example – tours4tips.com
Because the guides only survive if they make good tips, the tours are (usually) excellent.
For the first time I saw offered in Valparaíso, Chile Trekking for Tips.
A minimum charge is fixed at the beginning, depending on the number of people in the group.
After that you can tip the guide additionally if you like the experience.