Since Roman times, this city has been one of the most important in the Iberian Peninsula, so it's no surprise that in every corner you look you'll find stuff to do in Leon. Most of the places to visit in Leon are clustered inside the old wall, which remains largely intact. Within it, we can find Leon attractions as interesting as the cathedral, the Church of San Isidoro, and Plaza Mayor.
Off the beaten path, Piazza del Grano is not usually included among the list of attractions in Leon, but it's well worth a visit. It is a square stone with a large fountain in the center and a tree, surrounded by small arcades which hold bars and restaurants. It's one of the best places to enjoy summery Leon activities drinks on a terrace on a warm night.
But the things to do in Leon are not limited to the interior of the old town. Outside is the Parador de San Marcos, which is not only a five-star hotel but is also one of the most beautiful things to see in Leon. Take advantage of the walk to San Marcos to visit MUSAC (Contemporary Art Museum of Castilla y Leon), not only for the exhibitions inside but for the building itself, which won several architectural awards worldwide.
You should have no trouble filling your list of what to do in Leon, but if you do need help, look at the Minube community's recommendations.