Love Venice. Been 16 times throughout my life and never ran out of things to do. Went most recently with my toddler to the Biennale art festival and it was utterly amazing.
Honestly think its the most beautiful place in the world.
My tips about it are:
NEVER go in july or August. Best times are late may or september. It gets ridiculously hot and crowded in mid summer and is just not worth the hassle. Im talking body to body levels of crowded.
Buy a boat pass for the 'buses' which are actually boats, which covers the whole length of your stay. They look complicated but you will easily get used to how they work and they are the easiest and cheapest way to get about. They are much much easier than walking as remember you cant just cross the road on the main island, you would need to walk till you reached a bridge and it can become very complicated and very easy to get lost. A journey which looks like it would take a few mins on the map can end up taking an hour. So if you want to go somehwere specific its best to get on the boat bus/Vaparetto. Buy the pass because you really get stung when you buy individual tickets from stop to stop. Walking can be lovely but as i said its not ideal if you want to be somewhere at a certain time.
Dont eat in San Marco square or on any of the side streets leading off it unless you are prepared to pay a fortune. If you sit down at one of the cafes in the square they will charge you a flat rate for even sitting there on top of what you pay for actual food and drink. The flat rate when i went last year was 10 euros. It is worth it if you can afford that, especially at night when the bands play, theres nothing quite like it but do be prepared for it to be ridiculously expensive.
The menus on the main island are usually changed over past 6pm to become twice the price so if you are on a budget stick to lunch out.
Also if you are on a budget or have children the best place to stay is on the Venice Lido. That is an island opposite the main island and only ten mins away max by boat. (covered by the boat pass) It also has a lovely beach good for kids. The food on the Lido is also much much cheaper and theres a big supermarket.
Public toilets is a good one to mention. They all cost money so keep change on you and its a good idea to find out where they are on a map before you set out as you dont want to have to pay loads of money to go to a cafe just to use the loo.
Do get a map. Its not intuitive even though its a smallish place, you will get lost if you just randomly wander and the street signs are notoriously bad.
If you have some money to burn go for a drink or even a meal if you are feeling flush, on the roof terrace or the Danieli Hotel. Amazing view!!! Very expensive tho.
The bridge of sighs is a must see but easy to miss. Its next to the Doges palace and you can see it from the bridge opposite. You can go over it if you pay to go in the Doges palace. The Doges palace is very beautiful and worth a trip if you have time.
The Guggenheim art museum is also a must see if you like art. Thats off the beaten track a bit. Even if you dont like art that much its a lovely building to go in and chill out in the small garden and cafe.
Arsenale is a wonderful area to wander round. Its the old military barricades. The cafes are a bit cheaper round there too.
If you have time its really worth visiting some of the smaller islands (they will be covered by your boat pass but do take a while to reach) In my opinion the best are Murano which is the lace island. All the houses have colours which was originally instead of numbers and its so beautiful. Torchello which is the oldest island and was originally where people had settled but the was ravaged by the plague. It has a very calm atmosphere and the oldest bridge over the canal. Called the devils bridge! Also has two churches which are interesting because one is in a western style and one in an eastern style and they are next to each other. Wonderful and cheap cafe there too with amazing pizza, play equipment for kids and very friendly goats!!! Cimetero which is an island that is a cemetary which doesnt sound very nice but its incredibly beautiful.
I personally dont like Murano the glass blowing island as its stupidly expensive for no reason and theres not actually much to see. You can see glass blowing around the rest of Venice as well. But i guess if you have time its one of the 'places to go'