Our cruise next month will be our fifth on Constellation, and two of the four Connie cruises we've already taken have been to the Baltic. The ship is our favorite in the X fleet and the Baltic is an incredible itinerary - as evidenced by our having done it two years in a row. It's an incredibly port intensive cruise but every port is well worth the effort.
With the exception of St. Petersburg, every port is tourist-friendly, accessable (walkable, or has easy to use public transportation or hop on/hop off tourist bus service) and easy to see on your own. In St. P organized tours are a must, whether you opt for ship-sponsored tours or book on your own through companies like Red October or DenRus - both of which we've used and both of which are excellent. I'd recommend you purchase a couple of good tour books (like Fodor) for Scandanavia and the Baltic and read as much as you can before you leave home. Also, you'd be amazed by the wealth of information available to you just by reading the Ports of Call forum for each port on this and other similar cruise forum websites. The forums contain invaluable information posted by people who have already been where you're going and you can learn a lot.
Re packing for a two week cruise - pack for 7 - 10 days and make use of the ship's valet service; it's quick and reasonably priced. With three in a cabin, storage space will be at a premium so try to limit those "just in case" items you probably won't really need. Keep everything color coordinated so you can mix and match and keep shoes to a minimum. Also, it's not a crime to wear something twice - or heaven forbid, even three times!
Enjoy your cruise!