This is not a first cruise for anyone. It's a cruise for people who've seen a lot, and want more. The "river" between St. Petersburg and Moscow is a combination of lakes, rivers, and canals. Near St. P, it's mostly uninhabited area. Near Moscow, it's more inhabited. Of the 5 stops on the "river," only two (Yaroslavl and Uglich) are real towns. Kizhi is fabulous, as a recreated ancient community. Mandrogy is an artisan village, and fun (and expensive if you shop), but for tourists. The monastery stop was interesting, but due to fog, we were shepherded past any vendors and didn't see a real town.
St. Petersburg and Moscow were fabulous - and exhausting! You walk your feet off. It's worth it, but just be warned. The worst crush was The Hermitage in St. Petersburg - and I was there in a shoulder season! But the fabulous art is incredible and not to be missed. As are any of the St. Petersburg tours, and definitely Moscow by Night. The Kremlin (included) tour was very good, but I wish I'd done the optional tour that included the carriages, jewels, dresses, etc. We saw St. Basil's in the afternoon, which will NOT get you good pictures. You need to be there in the morning.
Best souvenirs: Uglich. We skipped the tour and just shopped for 3 1/2 hours. Do not try to buy souvenirs in Moscow; for some reason, they are scarce and expensive. At the St. Petersburg port, there is a souvenir shop, which is better than nothing.
Small but exquisite. Cabin attendant Olya was fabulous. She even cleaned my hairbrush!