In Paris, what I really like to do is wander through the streets and markets...it's a cool city. I love the Latin Quatre for it's bookshops and the small food places you can enjoy. Musee D'Orsay is a fabulous museum, and much more doable than the Louvre (which is amazing, but not necessarily worth it for such a short stay).
London - there is so much to do (and as I live here, I know it better). One new thing that has come to London is the cable car ride over the Thames. It's very cool and much better than the London Eye. If you are in London Thur - Sat, I would hit Borough Market for lunch - meat eaters should try out the chorizo sandwhich guys, non-meat eaters should go for the Raclette cheese stand (I think they are still there). Definitely do a hop-on-off bus tour - you can use the ticket for 24 hours. And I love, love, love the London Walks (www.walks.com) - have yet to go on a bad one, and I go every year for at least one (which means about 9-10 walks). St Pauls is a great church to visit, and while insane, you should check out Oxford St & Regents St. If you can, see some music at St Martins in the Field, a small church in Trafalgar Sq - it's so quaint and fun. For food, I'd check out www.toptable.co.uk for reviews & to make reservations. Have fun!