Get out and about and see the country! You'll not really see or experience England if you stick to the big cities!
My best friend came across from Canada a couple of years ago for three weeks. We spent five days in London and did the touristy stuff, saw Buckingham Palace, the Towers of London, the National Art Gallery, took a open top bus tour, went on a river boat, the London eye etc.
If you want to do Paris - use London as your base, and then it's just a quick and cheap trip via the Eurostar to the centre of Paris (you could even throw in a cheeky day trip to
Disneyland Paris, which is about 45mins out of the city by train).
We then travelled around a bit - we went to Edinburgh for two days, and drove up to Fort William through Glencoe for the weekend... if you want to do Scotland by the way, Glencoe is stunning. It is prettier in winter though (when the roads aren't closed!).
And then we travelled back down, focussing on the smaller towns and villages - where you'll really fall in love with England... little cobbled streets and cottages with thatched roofs
For smaller towns with tons of history, try
http://www.theguardian.com/travel/2008/aug/24/shortbreaks.uk . You can barely throw a stone without being close to a castle, so maybe pick a couple of small towns to visit and go visit the local castles and historic houses! I'm biased, but I love Herefordshire... Hereford itself is beautiful, but there are some gorgeous towns close by like Hay-on-Wye which is famous for its books!
I second Durham and York - both beautiful! I also love Cambridge!