We stayed in Vigo and just went for a hike up the hill to the citadel. Easy to get there, not too far, but it is a climb. Nice grounds, great view, restaurant there (we didn't eat there though). There is a fish market that was suggested as an interesting place and also a museum of modern art, but we didn't check those out.
Or from Vigo you can go to Santiago de Compostela to the cathedral. It is about 90 minutes by train