San Gil sells rafting on three rivers and they are genuinely different trips. The Fonce runs through town and is the family one. The Chicamocha is bigger water in a spectacular canyon. The Suárez is class V and is not a thing to do on your second day.
Colombia Rafting Expediciones is the specialist — rafting is the core business rather than one line on a board of twelve activities — and they also run hydrospeed, caving, canyoning, paragliding and a day out to Barichara, the colonial town up the hill that is the reason half the people in San Gil are actually there.
Carrera 10 in the middle of town, open every day 08:00 to 19:00. One phone number, and it is the WhatsApp.