Last February I had the chance to spend just one
day in Venice during Carnival, and it was memorable. It was a fantastic
pageant, and even though the city was wall-to-wall with people, it stayed
civilized. If you can squeeze into the Florian Cafe on the Pza. S. Marco,
it is a nice place to warm up with a spot of tea and a sandwich. Once
inside it's not that crowded, and the pageantry of people and costumes
inside is great.
Hotel rooms are obviously at a premium, so plan accordingly. Since my jaunt into Venice that day was a last-minute decision, we decided to drive to Padova, park the car near the station, and then take the train into Venice. This worked out quite well, since there were trains about every hour back and forth. This commuting idea wasn't any hidden secret though-- the trains were standing-room only and especially packed leaving Venice in the evening.