I always enjoy the walk along the river to Ile de la Cite/notre Dame. Key for me on arrival day is to keep moving, and preferably outdoors.
"Opera House" can be two things -- the new one at Place de la Bastille, which I don't think is interesting, and the absolutely magnificent old one (aka Palais Garnier) at Place de l'Opera. That one is definitely open for tours -- and it's magnificent. Highly recomended.