Yes, Hermitage is a must plus the Winter Palace.
Apparently the Hermitage is so huge and crowded you don't see much, but you have to decide if that's what you want. We went to Peterhof, which was incredible, but we had to queue ages to get in. You need to buy a permit to take pictures or leave your camera in the 'reception'. We gave one of ours to the rep for safe keeping and just paid for one permit. The city tour was very good with a nice lunch in an old palace hotel. Some friends went to the Ballet and others to the Folk lore theatre and Folk lore seemed most popular. Our other trip was to Pushkin and Catherine's palace which was also very beautiful. If you haven't been to Stockholm go to the Vasa museum, it's spectacular. It's walkable from town if you are fit.
We were in St Petersburg September this year. Ships dock in the commercial port and it is approximately 2 miles to the dock gate and then a further six (I think) to St P. The only people allowed into the port are those with a licence, ie local tour operators, etc. However, a new cruise port/terminal is being built, which should allow cruise passengers easier access into the city. Hopefully, they hope to have this completed in time for the 2009 cruise season. If you wish to 'go it alone' you will need a visa although our guide in St P did tell us that the government are considering a 3 day visa for cruise passengers, but there is no indication as to when this will happen.
If you book a ship's tour or one with an independent operator, you are covered by the group visa. We booked a two day tour with Alla Tour (www.alla-tour.com) and they were excellent. Our guide, Marina, was brilliant. She knew everything about everything in St P!! The cost for the two days was $300 pp, which was cheaper than the ship's excursion. We visited The Hermitage (early entry before the crowds), Church on the Blood, St Isaac's catherdral, Yusupov's Palace. Catherine's Palace, St Peter & Paul Cathedral, Peterhof. Alla will also arrange evening visits to the ballet/fokelore performance. To take everything in you really need more than 2 days, but, with the time available we found the tour excellent. Hope this helps
As others have already mentioned I think the pre arranged tours with Alla Tours or DenRus are good value for money. You get to choose what you want to see and the guides are excellent. I visited St Petersburg last year with Fred Olsen and went on a wonderful evening tour organised by the ship, entitled 'Musical Evening at the Hermitage.' The tour was limited to 120 people and we were invited to visit the Hermitage when it was closed to the public. A guide took us through the numerous staterooms of the Winter Palace and we were also able to view masterpieces from the collection of Western European art. We were allowed to stroll at leisure and take numerous photographs of the treasures, with the added bonus of no crowds. At the conclusion of the tour, members of the State Hermitage Orchestra performed a 45 minute concert in the Italian Skylight Hall. The evening ended with champagne overlooking the Grand Staircase of the New Hermitage. An expensive evening at £100+, but a truly memorable and moving experience. The sea plane flight in Oslo is another amazing trip if you get the chance to visit that city. Happy cruising, Jan .
If you do go to stockholm do visit the vasa museum. Much cheaper to do it yourself. We were on Jewel may 2008 it berthed just next to the landing stage for the water taxi. Buy an all day ticket and you can hop on and off as you like. we caught the changing of the guard and walked around the old town and then went on to Vasa.
As someone with limited walking ability, which tour of St Petersburg would suit to get the most out of this tour.