The water taxi from the airport isn’t the cheapest option (that would be the bus) but it’s definitely worth paying for and gets you near the northern shore of the island.
As above, get a Vaporetto day pass and take that everywhere (very regular, fast and great views).
My favourite restaurant ? There’s an amazing pizza place opposite the cinema – full of locals and families…
Just walk around and get completely lost – there’s nowhere else like Venice, so just wander around randomly for a day and take it all in.
Also, get the ferry to the local islands. I recommend the furthest and quietest (about 45 minutes away) called Torcello – there are buildings there from the 6th century ! If you’re on the laguna in a boat when foggy with sunshine, it’s an incredible experience !