If you do the half day or full day car or van THROUGH Celebrity, you do not need a visa...
We didn't do this...but, I recommended to my folks STRONGLY that they do a private car both FULL Days when they're on the Constellation June 19...
For them, it makes great sense...they're 77 and 80 and don't exactly walk at the speed they did when they were much younger (although they don't get around too badly and are taking ship's excursuions everywhere else)...second, none of the regular tours visit the Synagogue and this would be of more interest to them than all the churches...also, it saves all the time of 40 people loading onto and off of buses at every stop...
But, my mother doesn't want to do this...she wants to be on tours with OTHER people--they're just too "social"...The other alternative would be a regular tour one day and a private car the other...which is what they're leaning toward doing...in which case they'll do the full day tour that goes to the Peterhof and Yusopov...and the other day, they'll have the private car take them to the Hermitage, the Synagogue and whatever else they can fit in--on their schedule...
They would have compromised at the full day vans if they had 4-6 others to go with them...but, it's too logistically difficult for them to identify and coordinate unknown tourmates...
