We were recently in St. Petersburg and here are a few tips:
Hotel- We stayed at the 3Mosta Hotel which was wonderful, close to The Church of Spilled Blood and The Hermitage.
The Hermitage lunch- We carried in a small pack and took in lunch food from the Stole restaurant. They are a restaurant chain with many locations around the city. They sell great pastries with various fillings (such as egg and onion, salmon etc) by weight. We sat at the internet cafe on the ground floor of The Hermitage and there was no problem eating our food or drink.
Restaurants- We particularly liked the Zoom restaurant and the Cafe Botanika that Rick recommended in his book. We also loved the Georgian restaurant about a block from the 3Mosta Hotel (they recommended it). It was so good, we ate there twice.
Bakeries- our favorite bakery was the Bize. There are several locations.
We loved St Petersburg and found the locals very pleasant and helpful.