Brussels would be a great place to stop. You can easily take a train north from Paris to Brussels (I recommend the Thalys) and then take the Eurostar from Brussels to London. Very easy trip and there's lots to see and do in Brussels--wish I'd had more time there. The high-speed trains are worth it just for the experience
It's a great way to see Europe, and taking the Chunnel is one-of-a-kind.