I went on a 12 night cruise around Italy this June. I took my D80 with a 16-85mmVR and a 50mm 1.4. I was glad I left the heavy stuff home. If you're doing tours, you don't really have the time to set up the classic shots that you think you're going to get. Also you are almost always fighting the crowds. I took the 50mm f/1.4 to use for low light indoor shots, and found taking photos were not allowed in many places! If I were you I would take your D300 with your 24-70mm and your 35mm. The towns are small with small streets, and I didn't feel the need for a longer lens. I found that the 85mm end of my lens covered everything good. If you feel the need to zoom in on a statue or some thing, just use the 24-70mm, and when you get back, do a crop on the area you want. With the IQ of Your D300 and 24-70mm you should get some decent results. If your trips goal is to satisfy you photography needs, then by all means, take all your gear.If your going on the trip to see the sights and enjoy the beauty of the people and culture, then travel light. After you've been walking around all day, that heavy camera bag wears thin. Enjoy your trip, and the best of luck to you.
JMHO
ciao, Ed