Which of these London sights best combine with a half-day trip to Greenwich

Any of these can work. I suggest returning in the evening to stretch your day and also view London from the Thames at dusk.

We found we spent much more time visiting Greenwich than we had anticipated. Very interesting and lovely day. I think I'd choose attending mass at Westminster, then heading to Greenwich. Make sure you take a picture while you are standing at the Prime Meridian Line!

We spent a lot of time at the Churchill War Rooms and Tower of London, so I'm not sure I'd try to tackle either of those in the same day, because we don't like to feel rushed trying to get to the next big thing!

Last summer we did the Tower of London in the morning (were there at opening until about 12:30) and then took the boat to Greenwich for the rest of the day and thought it worked out well. We spent more time in the Churchill War Rooms and I don’t think we would have had enough time left to also do Greenwich on the same day.

Think any would work. We did the National Gallery one morning, and then our trip to Greenwich that afternoon.

I like the Tower of London at opening, Uber boat from Tower pier, then Greenwich. In fact, when I share London with friends and family as their unofficial guide, this is my usual combination. I like the fact that early arrival at the Tower means no line for the Crown Jewels. The boat ride gives a restful way to sit, refresh and enjoy the river scenery. Greenwich has many interesting sights. Don’t miss the Painted hall. I personally enjoyed it more than the Sistine Chapel.

I’ve done this combo several times now and it has always worked quite well.

Last November we arrived at the Tower of London at 10:30 and didn't leave until 4:30. Depending on your interest it could be a full day.

And in Greenwich, don't miss out on the Queen's House for the Elizabeth I "Armada Portrait" and the Tulip Staircase in particular.

Thank you for all the good advice. It does seem that combining Greenwich with Mass at Westminster Cathedral has the advantage of a predictable ending time for the Mass. But going to Greenwich after Tower of London is a shorter boat ride.
Carol now retired, since you have done this with visitors more than once, what time would you say is typical to finish at Tower of London if get there at opening?
I appreciate all the tips on what to see in Greenwich as well.

The time ending at the tower can vary, but usually about 2.5 hours. We take the boat to Greenwich and eat lunch there.

A lot depends on the individuals and what interests them.

Go with The Tower of London, but don’t stay too long. The earlier you can get started and get to Greenwich, the better. For the Tower, we’ve mostly seen the inner courtyard exteriors, and enjoyed Yeoman Warder (Beefeater) tours. The last visit, we popped into the White Tower for a few minutes for a quick glance at some of the displays inside (lots of stairs if you stick around), and have always rushed through the Crown Jewels display, riding and re-riding the conveyor belt past the cases of crowns and other royal jewels at least a couple of times.

Be sure to be at Greenwich at least a couple of minutes before 1:00 PM, ideally outside the back of the Maritime Museum and looking up at the big yellow ball on the pole atop the Royal Observatory. At precisely 1:00, the ball drops down the pole, and “keeping your eye on the ball” is what sailors did for a long time to synchronize their ship’s clock. Interesting and practical tradition, and it pre-dates a baseball batter keeping their eye on the ball.

Good to know, thank you!

Wouldn’t you have the most time in Greenwich if you took the light-rail connection?

Don’t overlook the Old Royal Naval College (a frequent spy film shoot location) and its many elaborate interiors to be seen.

I found the observatory crowded and contrived.

Really for me Greenwich deserves a full day - anything else and you will have to prioritise what to leave out. There is the Cutty Sark, the Royal Observatory, the National Maritime Museum, the markets (both craft and food), and the Old Royal Naval College with the beautiful Painted Hall.

And the best way to get there and back is one way by cruise and the other by the Docklands Light Railway, which will take you back through Canary Wharf on an elevated railway, apart from the tunnel under the river.

I realize we won’t be able to see everything in Greenwich in half a day, but that is the best I can do with the other places I want to see.
I thought about taking the DLR to Greenwich and then the boat back. From Tower of London, is the DLR to Greenwich faster than the Uber boat? Is the DLR station as easy to get to from Tower of London as the pier?

DLR takes 22 minutes from Bank to Greenwich- https://www.railwaydata.co.uk/timetables/tfl/PDF/DLR2%2009aug25.pdf

Alternatively it is easier and closer to join a Beckton train at Tower Gateway Station and do a same platform change at Shadwell or Limehouse into a train from Bank- https://www.railwaydata.co.uk/timetables/tfl/PDF/DLR1%2001sep25.pdf

The Thames Clipper/Uber Boat takes 21 minutes from Tower or 43 minutes from Westminster- https://www.thamesclippers.com/plan-your-journey/timetable

So you could use one on the way out, the other on the way back.

Personally I favour the idea of Greenwich on Sunday after Mass, to avoid you feeling rushed or that you have short changed the not inexpensive Tower or CWR. Either walk or take a bus down from the Cathedral to Westminster.

Thank you for the specific transit information; that helps quite a bit.
And I do see your point. Leaving after Mass should get us to Greenwich by 12:30 it seems. The biggest concern with leaving after Tower of London is that I will have at the back of my mind that we should be leaving by a certain time. If I visit Greenwich after Mass on Sunday, then on my Tower of London day, II would probably visit St Paul’s Cathedral after Tower of London. Unless I’m overlooking something, the time pressure seems much less with that combo.

I think your choice makes the most sense, Greenwich really takes up a whole day so you would want the shortest activity in the morning. There is so much to do there- even just walking around the park is lovely (especially in May).

I think our Lake District friend has the best idea too. The predictable end time for the mass, plus the longer scenic boat ride to Greenwich, and enough time there to appreciate it. Take the DLR or other fast transport back into the city when you're tired at the end of the day. The only downside is that Greenwich may be more crowded on the weekend -- but so will any other pace you want to visit, and with Greenwich you can move around outside and make choices.