Avila is a charming, small city (or more aptly, town), but the only major attractions there are the walls and the cathedral. Don't go unless you want to visit a very quiet place, or you are a die hard fan of Sta. Teresa de Avila, or San Juan de la Cruz. The train to Avila from Madrid takes about 3 hours, so if you get bored easily, stay in Madrid. In my opinion, though, it is a nice day trip.