I agree on the Cueva Tito Bustillo. The art rivals Altamira, and it's the real thing. Also agree on Lagos de Covadonga, and note the shuttle isn't mandatory, but you have to be at the gate at the basilica before 8 AM. But it does sound like the drive is what you want to avoid, so maybe the shuttle is better.
The Ruta del Cares is spectacular, but could make for a long day, especially if you elect to hike out and back (4x4 shuttles are available if you prefer one way). It's completely flat and level, except for the first 1 km or so from Poncebos, which is somewhat steep.
If the Ruta del Cares is too long of a day for you, another option is to hike to and/or from Poncebos to Bulnes, you can also hike one way and take a funicular for the other leg. The hike is steady uphill, but not exceedingly difficult. Parking at Poncebos can be limited, and depending on personal comfort, the drive to Poncebos might be a bit intimidating, a bus is available from Las Arenas to Poncebos. (Poncebos is also the start of the Ruta del Cares). Check Google street view and decide based on your comfort level and preferences.
The Lagos and Poncebos are much closer to Ribadasella than Potes, so if you want to keep your day to 8 hours, they might be better options than Potes. On the hiking, the hike to Bulnes is steady uphill but not excessively steep, the hiking in the Lagos is mostly level, and the Ruta del Cares is entirely level except for the first 1 km or so from Poncebos.
As far as a "very old" village, you may not find much within 1.5 hours of Ribadasella. Bulnes is an old, stone village, but since the funicular, is somewhat gentrified with farmhouses turned into cafes and restaurants. Santillana del Mar is old, but also populated by tourists. Peñalba de Santiago certainly fits the bill, but is closer to 3 hrs (each way) from Ribadasella and the drive may be in parts what you want to avoid.