Disneyland Paris is not as easy to get to by train as is sometimes implied and I agree with Andrew when he says you'd need to stay over at least one night - it is hard to make good value on a ticket this way as you would have 2 half-days though.
Flying is good as long as you don't try to pick up a hire car from Hertz at Charles de Gaulle airport - we did this once now we take a taxi. You can fly out of all London airports to Paris (some use Orly some use C d Gaulle). Be warned DLP is not a cheap trip at all. We find we can visit WDW in Florida for not a great deal more than a long weekend trip to DLP especially if you stay in a DLP hotel. It is ok for us Brits looking for a bit of a Disney fix but if you've been to WDW it may well be quite a disapointment to you and I wouldn't go out of my way to visit it if you have other plans in London.
Other attractions:- the walk along the river past Tower of London and onto Tower Bridge is good. The area is pretty touristy and there is lots of tourist stuff to do - The London Experience is by the tower and you can visit the Crown Jewels inside the Tower. The view from the top of Tower Bridge is great - you can get a similar view from a river view room at the Thistle Tower hotel aswell and this is a great hotel although not cheap it is not outrageous either. Another hotel I've had recommended to me which is slightly pricier is the old GLC building which is now a Marriott hotel. A friend stayed there before Xmas and voted it excellent.
You can visit the Cutty Shark ship over in the London Docklands - easy access from central London on the new Docklands railway.
My kids (8 and 10) absolutely love the Natural History Museum - it is a fantastic building just from the outside and inside is so much stuff you can spend a whole day there. Particularly good are the Dinosaur exhibit and the Volcano/Earthquake area.
Nearby is the shopping area of Knightsbridge which is always worth a look even if you don't buy anything - Harrods is here and the toy department is very entertaining. Hamleys - the toy shop - is great too although never discovered how to get out without buying something yet.
I'm not sure how far out of London you fancy going but Stratford on Avon (Shakespeare area) and also Warwick Castle would be worth investigating, also the Cotswolds are nice for a few days in the country. Portsmouth has a lot of naval history and of course the Mary Rose.
Angela