I found the Alexander Palace much more interesting than the Catherine Palace - far more intimate and 'lived in', and more material in English. Interestingly, on the day I went there were huge queues for the Catherine Palace but hardly anybody at the Alexander Palace. However, you do need to allow plenty of time for getting there, unless you are prepared to pay the earth for taxis. The trains to Tsarskoye-Selo are not that frequent (at any rate on Sundays) and the palace is a fair walk from the station.
I only had a morning for the Peter and Paul Fortress (flight to catch). It deserves more. There were huge crowds in the Peter and Paul Cathedral from the moment it opened, so difficult to adopt a properly reverential attitude to all the tombs.
Good luck!
Ann