If we only have time for hiking one day, where on the El Camino do you recommend going?
Is the goal to take a short walk on the Camino to get an idea of what it is like and see some nice scenery? There's a bunch of options, and what you choose depends largely on your transportation and where you will be.
I really enjoyed the section just before OCebreiro - it is a proper hike on trails, up a steep incline, and onto the neat town of Ocebreiro. Villafranca de Bierzo has good transport (Alsa Bus runs frequently), so you could walk from there or if that is too far, maybe take a taxi up the road a bit to shorten the distance. You'll have a mix of road walking and trail walking, and the route goes up a nice river canyon.
Be sure to walk with a companion for safety.
Santiago to Negreira on the section that goes to the coast (Finisterra). Frequent bus to come back.
Benny - You often make a comment about walking with others for safety. Have you walked the Camino? Did you have issues? I ask because violent crime in the Camino is amazingly low - even petty theft is pretty rare. Tons of people walk the Camino solo each year.
There is no security issue on the Camino. You can hike alone.