Congrats (perhaps...surprises are sometimes unpredictable).

Surprises: hard to pull off, need a lot of planning to do well, and almost always should take place indoors (controllable), and to allow for perhaps ...anything to happen!

I suggest that you make a res at a restaurant or bar--take them all to dinner, have aperitifs...and with the Champagne or the vodka cocktails...pop the question. The Grand Europa hotel comes to mind. There are now some fantastic restaurants in St Petersburg. I suggest not on a boat, not on a river cruise, definitely. On land. Dress up. Adult...transitioning in to marriage.

Don't, whatever you do, think of having it outdoors. What if it rains or is too hot--what if she is tired and wants to go back to the hotel? What if, what if...and especially don't do it late at night when everyone is whipped.

At a restaurant--everyone will be there at the table, and if you pop the question early and get that over with, you will not have that nervous thing where she knows something is going on and wonders why you are acting weird and wonders if you are having an affair because you are so nervous and squirrely.
A dinner: it's like a play. You are at a table, people are dressed up (your girl will want to look her best, not blown around on a river bank...)...and having fun.
You get the help of the waiter/manager.
They bring out the Champagne, and just as it is poured, you say, " I have an announcement'...cheers all around, she says yes, you put the ring on her finger (or you get down on your knee, hard to pull off.)
Then you all have the most wonderful and happy dinner.
Outdoors on a bridge or beside a statue...'remember when we were all standing on the bridge and it was raining and so windy or and...' does not have such a good sound as...'the Champagne cork popped and...and then we had a fantastic dinner'...that would be happy.
Also--as a future husband--it is great to pull this off, manage it expertly, and look in control, like you are a person who is reliable and accomplished and a good provider. She will say yes. (beside a statue...no accomplishment there, anyone could do it)
I wish you well, and be sure to let us know how it turned out.