Cascada de Juan Curi is the waterfall trip most often sold out of San Gil. It sits south of town on the road toward Charala, with a short trail through humid forest to the falls and a pool below the main drop.
You can visit simply to walk and swim, but many travellers come for waterfall rappelling. Go with a reputable operator if you plan to descend the falls; the setting is beautiful, but wet rock and ropes are not the place to shop only on price.