Italy in May -- what to do with 3 days?

Frankly, I would allocate those three days to Venice and Rome. (And it's better to think in terms of nights for planning.) So I would do three nights Venice, five nights Rome. (Keep the Venice Lido as a back pocket option.)

Rome has some lovely walks as well as some easy day trips. You could go to Ostia Antica in the morning one day, walk around among those ruins, then hop back on the train to go to Ostia Lido (beach) for the afternoon.

Another lovely day trip from Rome is Orvieto. About an hour by train, lovely hilltop town. TONS of charm and good food and great for walking around.

Train to Lucca and stay with a side trip to Pisa and a side trip to Viareggio for the beaches, the Liberty style architecture and the pine wood parks.

Frankly, I would allocate those three days to Venice and Rome. (And
it's better to think in terms of nights for planning.) So I would do
three nights Venice, five nights Rome.

Michael's suggestion gets my vote as well. There is a LOT to see in Rome, and some of the extra time could be used for a day trip. Extra time in Venice will allow you to explore another island or just enjoy some relaxed exploration of its many nooks and crannies.

Keeping it to just one move between two bases will maximize the time you'll have to spend on fun!

yup, what they said

Split all of that time between Venice and Rome. They both have more than enough to occupy that amount of time. And you wont lose precious time going from one place to another -- and that always takes more time than one anticipates.

I think 5 nights in Rome is excessive, just in case Kate is one of those people (like me) who can't seem to warm up to it.

I would do 3 nights Venice, 1 night in Orvieto (to experience a peaceful evening in this lovely hill town), and 4 nights in Rome.

You can also take a bus from Ostia Antica to the beach (lido). I would do that rather than rush through.

If you don't warm up to Roma, go to Bolsena for the day, which has a volcanic crater lake with a beach. You could even make Bolsena a one-night stopover.

Two great options for long walks in Rome away from traffic noise are the Borghese gardens and the Aqueduct Park.

Thank you all SO MUCH for your thoughtful and helpful replies!

I'm taking your advice and changing Venice to 3 nights, Rome to 5 nights, and planning on doing day trip(s) from Rome based on how we feel then.

Thanks again! You all are wonderful!

Excellent decision. Remember that in Venice, if you walk away from the crowds around the Rialto Bridge and Piazza San Marco, you'll find quiet spots and picturesque canals.

I'm glad you decided on three nights in Venice. If you have time, use your vaporetto passes to visit the islands in the lagoon .... Murano and Burano are the two most famous but there are others as well.

Another favorite thing to do with a vaporetto pass is to just get off at some random place and wander. Even on really busy days, you can find yourself on an almost-empty canal. Get a glass of wine or a gelato, sit by the water and just soak up the atmosphere. You can always find your way back to Rialto or San Marco or to another vaporetto stop.

If you find yourself near the Accademia bridge near sunset, watch for a flotilla of gondolas coming up the Grand Canal from San Marco. I don't know if they do it every day, but we happened to catch it once and it was amazing. There were musicians in one gondola, and all the gondoliers were singing Santa Lucia.

And finally, be sure to take a vaporetto down the Grand Canal after dark. It's so pretty to see all the palazzos lit up.

Florence would be the obvious third option, you'll be going through Florence between Venice and Rome. Day trips on a short trip make no sense to me, why add more travel time? Stay near it is that you want to see. Two nights in Venice were enough for me, beautiful, but overrun in the daytime.