Don't insist too much about dressing up or getting her nails done
If you're both not used to dressing up fancy for a walk don't wear a suit
A good motive for dressing up will be a photoshoot (if you decide to go for a proposal during the shoot)
You can tell your partner that you are going to a fancy restaurant for brunch, lunch, dinner, or an aperitif, and the restaurant requires formal dress.
Ask yourself will she care if here nails are done? If yes, you can ask her friend to invite her to do their nails together, don't make the idea come from you!
If you’ve never before suggested she get her nails done, now is not the time to ask.
Plan a Backup Story
You planned place and time, now you just need to be there.
Have a back story for when your partner asks: Why do you want to take an early morning stroll at San Marco square? or why you want to get to this specific location at this specific time?
Make up a story! "There is this awesome restaurant I made a booking", " there is this beautiful view I saw on Instagram and want to visit", "I've booked guided tour"
Discuss with your local expert to choose the best backup story.
Disguising it as a photo session?
It's possible that your partner heard of this tactic and if a photoshoot is totally something unusual from you it might look suspicious.
On the other hand you are in Venice, which is already an unusual setting and its quiet natural you are willing to capture your romantic vacation.
It would be even better if it will be your partner who suggested hiring a photographer.
You can ask your friends occasionally to ask if you are going to book one of these very popular vacation shoots, so you can make it to be not your idea!
In my experience, it works well and gives us extra flexibility and the best reason for getting dressed up.
After the proposal was captured I always asked if they were suspicious and expecting the proposal. And if some were expecting it to happen soon or even during their trip to Italy no one would be able to predict the exact moment and how it would happen!