Peter - Last year I had a tourist day to myself on the Monday after the marathon. Personally I find walking the day after is great for the legs, but stairs are evil!
I wandered from the Arc de Triomphe to the Pompidou, stopping off for a coffee on the way. Then I spent some time in the Pompidou before wandering past Notre Dame and grabbing a bite to eat in a cafe in St. Michel. I then spent some time in the Pantheon. After that I walked to the Eiffel Tower (didn't go up). All easy paced and good for the legs, but maybe more distance than some would fancy the day after the run.