Most museums are closed on Monday.
Museums open every day
Doge's Palace
Museo Correr + Archaeological Museum + Royal Rooms + Library
Glass Museum (Murano)
Museums open on Monday
Ca' Rezzonico xTu
Fortuny Museum xTu
Goldoni's house xWe
Most attractions open at 10:00. Many museums have last admission 30 minutes before closing.
Open early
Doge's Palace 9:00
Close late
Doge's Palace 19:00 (last admission 18:00)
A cappuccino and a pastry make a great light breakfast. In most places, the price is higher if you sit at a table.
Cicchetti make a great quick lunch -- they can be very light or fairly substantial, and won't eat up a big chunk of your day and make you lethargic in the afternoon (unless you have too much wine!). But many cicchetti places don't offer seating. Some that do include the following, but seating is limited and you may have to wait:
El Sbarlefo (DD and CR) xTu-Sa
al Ponte CR/CA xM
da Fiore SM xMTu
Vecia Carbonera CR xM -- particular large seating area, unlikely to fill up
During a long day of sight-seeing, it's often nice to sit down for a while. Caffès with outdoor tables are good for this -- just remember that you'll pay more than at the bar: a small espresso might cost anywhere from E 2.50 to E 15 (on the Piazza San Marco). Some ice cream stores have outdoor seating and are self-service, so there's no extra charge for a table (notably the Gelato di Natura on Strada Nova SM M-Su).
Pretty much every toilet in Venice is clean and has sitting toilets (not Turkish).
There are a few public toilets in Venice; they charge E 1.50. But why bother? If you order a coffee (i.e. an espresso) at the bar of pretty much any caffè you can use their toilet, and it will only cost E 1.20 or so.
Museums have excellent toilets, and if you have the MUVE card, you can drop in at any time without paying.