I booked two nights at Ibis Lafayette which should be walkable from Gare de l'est where we arrive at 15:17. I was thinking that after checking in to hotel there may be enough time to go to Montmartre neighbourhood and walk around there for a bit for the main sights. Is that a reasonable plan to get started or not enough time?

Then on the full day focus on the places along the Seine from the Ile to the Eiffel Tower, and from Place Concorde along Champs Elysees to Arc de Triomphe. We don't intend to visit any museums, though the Chapelle sounds interesting to go inside. Will probably get a Navigo Day pass to use buses (what routes?) along the Seine and Metro, or is the Batobus a better idea? We are undecided if we should get a carnet and a day pass, or a navigo week pass, and also not sure yet what the best option is (RER B, 350, or Roissy Bus) to get to CDG.