If you get sick of the organized sightseeing in Paris - treat yourself to a lazy looooonng lunch in a nice café in Montmartre.
One of the slightly lesser known attractions in central London is the <a href="http://www.iwm.org.uk/cabinet/">Cabinet War Rooms </a>, great if you're a history buff. In Dublin, you MUST have a pint of Guinness. Exported Guinness often has to be pasteurized so it NEVER tastes as good as it does in Dublin.
The Tower of London really isn't compulsory, it's up to you.
The Beer in Prague is goood..
Enjoy..