Long pants and closed-toe shoes (boots preferred) are recommended for horseback riding. You'll also likely want to bring an extra couple of layers or a jacket for that spontaneously changing Boulder-area weather. Just like any outdoor physical activity, bring a bottle of water and sunscreen. You may also want to bring snacks if you're doing a longer ride. Typically the barn or stable will provide you with the option to rent a helmet, but you may want to double-check before booking.