Ours had numerous shore excursions; we spent whole days exploring the cities we visited. In St. Petersburg there was a full day at the Hermitage, and an evening of opera at the Yusupov Palace, among other things. Might not specifically be your sort of thing, but there are definitely things to "do"; you don't just stay on the ship the whole time, unless you want to.
I have heard my dad and grandmother say this is the greatest art museum in the west that neither of them has seen. It must be wonderful.