I just read your July 7 post on Rome, Italy.
We are around your parent’s age.
That is a lot of fast moving in a new place.
We have been to London several times and my husband still enjoys being guided by a tour guide once in awhile for a down time.

Once we all became frustrated because things were not going as planned on schedule.
Tour guides keep you on track in your 60’s.

You know your family but while in Rome we are using
http://www.walksofitaly.com/
They give Rick Steve’s discounts.

Thanks Delaine for dining tips. As for S. Zaccaria - Giorgio, I finally found (at the place I should have looked first) that this route is covered by a Ferry, Ordinary and each trip costs Euro 4. Here's the link http://www.actv.it/en/movinginvenice/movinginvenice
Now am not yet sure if this is a separate ferry that ply this route or Line 2 for this section is classified as Ferry, ordinary but atleast this short hop could be done for a little lesser.

Thanks betsy for your suggestions on walking tours. However, I have lately started preferring audio guides inside the museum/monuments and be my own guide outside as that affords me much more flexibility and take stops or linger on at a spot as per our preference. That's why all this pre-trip prep :) But I really appreciate your kindness in suggesting us the alterntives.