If you are not a fan of crowds, I highly suggest you to avoid going to Venice during Carnival period, which is around mid February to beginning of March.
Personally the crowdiness really ruined the fun for me.
But if you do wish to go check out the festival, which is to be fair, quite fun, many people dress up, you should:
- look out for pickpockets!!!
- book accommodation early - although it's totally do-able as a day trip. Venice is tiny
- to save money, you should book accommodation not in the centre like San Marco - again, it's tiny, so other areas are just as convenient and beautiful
- pack your own lunch, first of all restaurants in Venice are often very over-priced. secondly there are just too many people, it's hard to get a seat