Remember the Ceremony of the Keys is held at approximately 9pm each evening.
At the British Museum you can see the real Rosetta Stone (behind plate glass) and a fibreglass copy which you can touch and get another tourist to take your photo with it. It freaked out one of my friends who had never been to the British Museum to see a photo of my family with our grubby hands all over a world heritage treasure. Apart from that - I think the 2 hours at the British Museum is time well spent.
A word of note: there isn't much to see at the Changing of the Guard -- but that's just my opinion. If you're going for the ceremony, get to Wellington Barracks (just around the corner from Buck House) and you can see the guard getting ready for the Changing of the Guard. Inspection, marching, music and all. You get a much closer look -- you can be within feet of them as they march through the gates. This could save you some valuable time in the morning.
When you are at Kensington Palace, it might be worth a pop over to Harrod's just to see the place -- it is phenomenal, and the prices are out of this world. However, it is fun to walk through the Food Hall.
IMHO, once again -- I would save the Hop/On-Hop/Off tour for the Sunday & Monday when you are going to be doing the bulk of your touring. The buses essentially take you to the great majority of the sites you're hoping to see. This will save you both time and money on using the London Underground. The Underground can be very, very confusing. What appears to be a direct route from one station to another is not always the shortest distance between two points. For instance -- it would appear that, in order to get from Euston Station to Euston Square Underground, one must take the Northern Line to King's Cross and transfer to either the Hammersmith & City Line or the Circle Line. In actuality, go out the door and cross the street. Same that it is quicker to walk the length of the city block to get from Charing Cross to Embankment instead of waiting for the train.