Several tour agencies in San Gil run white-water rafting on local rivers. A 10km run on Río Fonce (Class I to III) costs COP$45,000 per person and takes 1½ hours; experienced rafters can tackle the rapids of the Río Suárez (COP$130,000, up to Class V). Most operators also offer paragliding, caving, horseback riding, rappelling/abseiling, mountain biking, bungee jumping and ecowalks.
Gunpowder, lead weights, alcohol? That's a dream mix anywhere, and in Colombia it's perfectly legal. Tejo, a rural tradition with roots in Boyacá, is a loud and rambunctious pre-Columbian game in which 2kg puck-like weights (once made of solid gold called zepguagoscua, nowadays made of lead) are tossed to a clay pit to hit a metal ring known as a bocin, which is surrounded by ready-to-explode gunpowder-filled triangular pieces of wax paper called mechas.
Staff at Macondo Guesthouse in San Gil run a tejo night every Tuesday at 8pm. They block out a few lanes at Comite Municipal de Tejo and organize a guide to help explain the rules and make sure you don't blow anything up that isn't meant to be blown up! The tejo is free for hostel guests; nonguests are welcome when space permits and pay COP$10,000 each for the guide. Cheap beers are usually delivered courtside by the crate.
Although well-behaved foreigners are welcome other days of the week (Wednesday and Friday are best for getting a court), it's not recommended unless you're educated in all things tejo.