End of March will actually be a great time to visit St. Petersburg. Although the weather will still be cold and the days grey and short of sunlight, don't let this put you off from getting excited about your trip. One of the major advantages you will have is that you will be in the city outside of it's tourist season. Many of the must see destinations located around the city are simply packed during tourist season where as you should be able to have more intimate experience while touring the city and it's sights. Make sure to check operational hours of places you would like to visit... some will still be running on a shortened winter schedule, so make sure to do a little homework before your visit so you can maximize your viewing pleasure.
As to hotels, I am partial to the Marriott brand and always stay at the Pushkin Courtyard. It's a 30 minute walk outside of the main center and 15 minutes to the local subway and train station but the hotel runs a free and efficient shuttle service that will whisk you away to numerous points within the city and will pick you up and bring you back to the hotel... it's a great service that will save you time and money. The Ren property is nice and strives for an elite feeling, including a nice bar/restaurant but to me personally there is something a miss at this property, same for the other courtyard which is located 20 minutes off the city center in Vasilyevsky Island. With so many sights located all over the city, it's hard to say there is truly one hotel that will put you within touching distance of all sights.
The major sights and experiences of the city are documented but if you have any questions I am sure many people here will gladly give you advice. As to some of the less then traveled paths, may I suggest the following....
Vasilievsky Island... often over looked by most tourists, this island is full of interesting sights and history. When visiting get off at the Vasileostrovskaya subway station on line #3. When going outside you will be greeted with a tree lined street for pedestrians only, numerous shops here that can be found on Nevsky Prospect but cheaper and less crowded. Just up from the subway station is Smolensky Cemetery, a beautiful and enchanting site full of history and stories. For more info, http://en.wikipedia.org/wiki/Smolensky_Cemetery
Catherine's Palace, located to the south in the city of Pushkin is a well documented site to see... especially the newly opened Amber room. However, what doesn't get enough credit is the park that holds the palace... it's amazing and huge. A chilly day in March should have you as being on the few tourists within the park... give yourself a full day to see the palace and to walk the park, it's massive and full of picturesque sights and buildings.
St. Isaac's Church... skip the lines and save your money for going inside and instead just pay and visit the church for it's outside views... the view from up top the church is amazing, even on a cold and grey day the sight is one to behold.
Alexander Park.... go there before 12pm and experience the daily firing of the cannon, it's shot off at noon everyday.
Varshavsky station also known as Warsaw Station... a former train station now turned into a museum. I'm not a train guy but the endless amount of trains from throughout Russian history was impressive. There must be close to 100 trains to view, very impressive display.
... I could go on and on but I will save your eyes. So much to do and see in St. Pete, don't plan on seeing it all... just too much to see and experience.