Prices
As a tourist, you should expect to spend more than the ordinary resident. Foreigner museum tickets, for example, are more expensive in Russia, and yes, they will ask to see your passport. However, keep in mind that Russia is not the most expensive city in Europe, and that pricing is often fair. So be aware of the rates whether picking a restaurant, getting a cab, or purchasing souvenirs. The ideal option is to ask for recommendations from locals, but even a short search on the internet might save you money.
Russia, like the rest of the globe, is making the transition from cash to card, but it will be a long process. Learn about the ruble, Russia's currency, and bring a small amount with you when you go sightseeing.
Small companies may or may not accept credit cards. There may also be fees associated with using your card, particularly at ATMs, so get some cash before you go. Remember not to bring big quantities of cash with you to any destination.