Visiting St. Petersburg on our Constellation May 22 cruise. After reading posts on this board and others, have decided our best bet is to arrange tours through Red October. For those who have been there, done that - Is it better to do four half-day tours or two full-day tours? Would appreciate a list of the "must see - don't miss" sights, as well as a list of "don't bother, it wasn't worth it" sights. Thanks!