Roman aqueduct that used to supply water to Nimes. Fantastically well-preserved and an incredible feat of engineering. You can pay to go on a tour inside the water channel at the top of the aqueduct (well worth it – but book in advance). Also – possible to combine with canoeing. Paddle down to it (with picnic) and under it – and get out to admire. See Collias canoeing under Activities.
About 45 minutes away by car.