We are planning to do a Sunday Thames lunch cruise. We have to be at the Embankment Pier by 11:30 and are staying in the Chelsea neighborhood. I don't believe there is enough time that morning for any tours, such as Tower of London. We were considering spending the morning walking around and taking in sites close to the pier such as Trafalgar square. Any suggested sites or walking routes would be very appreciated.
You probably right about not enough time for any tour simply because opening hours for sites are often at 10am.
You say you are staying in the Chelsea neighborhood and are leaving for your river cruise from the Embankment pier.
Have coffee and follow the Thames River pathway. There will be a few diversions but easy enough walk from Chelsea to the Embankment pier. Give yourself a good 1.5 hours. Lots to see and admire. Parks, architecture, river POVs.
If not walkers take the Tube from Sloane Square to Embankment. 20 minute stroll from the station to the pier. If not a Sunday you could grab breakfast the Embankment Place Cabman’s Shelter. There’s a bench on the sidewalk path near Cleopatras Needle. Lovely spot to watch water traffic on the Thames.
Rick Steve has a Westminster walk. One of his shorter ones so you could combine with a meander thru Trafalgar and SoHo. Perhaps stop for a coffee and pastry in a soho cafe.
https://www.ricksteves.com/watch-read-listen/audio/audio-tours/london
Being a Sunday, keep an eye on tube line closures. The powers that be love to close tube lines, or portions of tube lines, on weekends for repairs. If that's the case, give yourself extra time getting to Embankment.
Additionally, keep an eye on any events happening in the general area that might impede your getting to Embankment. As an example, there was a large march yesterday in Westminister about
the rising cost of living. Most of these events happen on weekends. The Westminster tube stop is the one just before Embankment.
Anglican churches tend to charge to visit, unless you go to a service. Sunday morning might be the perfect time for St. Paul's, Westminster Abbey, or whatever interests you.