If you live in London, then going for a tour of the Houses of Parliament is one of those things you tend not to get round to. But if you do feel like doing something 'touristy' for a few hours, this has to be one of the most interesting options.
The tour comprises a walk around the House of Lords and House of Commons, in a small group led by (in my experience) a charming and knowledgeable guide. Ours (Trish) talked us through the history of the building and the site, gave us a basic chronology of the development of parliament, and threw in lots of entertaining anecdotes.
The building itself is impressive both inside and out, and learning about the daily operation of the place, and all the preserved traditions, is really interesting. Much of the interior is visually familiar - most obviously the House of Commons debating room, where you can inspect (though not sit on) the seats occupied by Cameron and all the rest. You can also stand on the spot where Charles I stood to hear his sentence, explore the place where two small boys hide while the Queen is robed, and see the damage done by the traditional knocking of 'Black Rod' and by the bombs of World War II.
In fact, there are lots of ways to get inside parliament, especially if you're a UK resident. You can queue up on the day to watch debates or committees, visit the archives, or go on a tour of Big Ben (this must be arranged through an MP or member of the House of Lords).
Tours run on Saturdays throughout the year, and during the Summer Opening (this year 29 July - 3 September and 19 September - 1 October). Adult tickets are £15, and can be booked in advance, or purchased on the day if available.