As RS said , "a dizzying array" and it is.
Visiting end of Sept. into Oct. Arr. mid-morning on Fri. 29th, lv to Florence on Tues. 3rd. Will want to see the biggies in St. Marks, maybe skip the tower and go over to San Giorgio tower for view. Guggenheim, and the islands museums for glass and lace. Mainly want to avoid the long lines for entry, aside from required security. Thoughts? ty
Single vaporetto tickets will eat you alive at €9.50 each. There are 24-, 48-, 72- and 168- hour vaporetto passes, but nothing for 4 or 5 days. You can choose between the 72-hour pass at €45 (plus individual tickets during the other hours of your stay) or the 168-hour pass at €65. You can take care of your transportation needs separately rather than buying a combo sightseeing + transportation pass.
This isn't to say you wouldn't benefit from a sightseeing pass, but that's not the only way to avoid ticket lines. Buying individual tickets will do that for you as well. You can book an entry time online for San Marco, and it's a good idea to do that. There's a fee for that service; the basic entry is free. I thought the Pala d'Oro and museum add-ons were both worthwhile, but as of last September you could easily pay the fees for those after arrival.
It's unlikely you'd run into a significant line at either the glass museum or the lace museum. Note that it takes considerable time to travel to and from Burano.
The Guggenheim is popular; it's a good idea to buy that ticket online in advance. I haven't heard of early sellouts, but I saw lines that might have been for buying tickets.
The Secret Itineraries Tour at the Doges Palace is quite popular on the forum. It sells out in advance, but I don't know how early.
TY for the great feedback. Much appreciated.