My favorite day trip is always to Greenwich by boat. Plan on a full day if you go. There is a lot to see in the area besides the the GMT line.
Greenwich, Cutty Sark and Canary Wharf are all on our list. Our Airbnb is in Limehouse and the Docklands Light Rail (DLR) goes directly to them. A boat ride down the Thames is also on the list.One of my favs as well. Great restoration of HMS (?) Cutty Sark right there, too. The trip is worth it even if you stay on the boat and return to London!
Thanks everyone for the suggestions. Much appreciated.